Oxygen Abundances in Solar-Type Stars

The absolute solar oxygen abundance,ε ⊙ = 8.80± 0.06, has been determined from various oxygen abundance indicators in different solar atlases, and a new method is proposed to test collision rate coefficientsfor the NLTE model of OI.Using effective temperatures derived from Balmer lines, oxygen abundances from O triplets in 83 solar-type stars within the solar neighborhood spanning a metallicity range of [Fe/H] = −2.3 ... +0.4 have been determined.NLTE effects are not negligible, especially in warm stars(T_eff ≥ 5800) with [Fe/H] ≥ −0.5. This revised version was published online in July 2006 with corrections to the Cover Date.     

RHESSI Observation of Atmospheric Gamma Rays from Impact of Solar Energetic Particles on 21 April 2002

The RHESSI high-resolution spectrometer detected γ-ray lines and continuum emitted by the Earth's atmosphere during impact of solar energetic particles in the south polar region from 16:00–17:00 UT on 21 April 2002. The particle intensity at the time of the observation was a factor of 10–100 weaker than previous events when gamma-rays were detected by other instruments. This is the first high-resolution observation of atmospheric gamma-ray lines produced by solar energetic particles. De-excitation lines were resolved that, in part, come from ¹⁴N at 728, 1635, 2313, 3890, and 5106 keV, and the ¹²C spallation product at ∼ 4439 keV. Other unresolved lines were also detected. We provide best-fit line energies and widths and compare these with moderate resolution measurements by SMM of lines from an SEP event and with high-resolution measurements made by HEAO 3 of lines excited by cosmic rays. We use line ratios to estimate the spectrum of solar energetic particles that impacted the atmosphere. The 21 April spectrum was significantly harder than that measured by SMM during the 20 October 1989 shock event; it is comparable to that measured by Yohkoh on 15 July 2000. This is consistent with measurements of 10–50 MeV protons made in space at the time of the γ-ray observations.     

Solar Cycle Variation of the Interplanetary Forward Shock Drivers Observed at 1 AU

Forecasting space weather more accurately from solar observations requires an understanding of the variations in physical properties of interplanetary (IP) shocks as solar activity changes. We examined the characteristics (occurrence rate, physical parameters, and types of shock driver) of IP shocks. During the period of 1995 – 2001, a total of 249 forward IP shocks were observed. In calculating the shock parameters, we used the solar wind data from Wind at the solar minimum period (1995 – 1997) and from ACE since 1998 including the solar maximum period (1999 – 2001). Most of IP shocks (68%) are concentrated in the solar maximum period. The values of physical quantities of IP shocks, such as the shock speed, the sonic Mach number, and the ratio of plasma density compression, are larger at solar maximum than at solar minimum. However, the ratio of IMF compression is larger at solar minimum. The IP shock drivers are classified into four groups: magnetic clouds (MCs), ejecta, high speed streams (HSSs), and unidentified drivers. The MC is the most dominant and strong shock driver and 150 out of total 249 IP shocks are driven by MCs. The MC is a principal and very effective shock driver not only at solar maximum but also at solar minimum, in contrast to results from previous studies, where the HSS is considered as the dominant IP shock driver.     

<Emphasis Type="Italic">In Situ</Emphasis> Signatures of Interchange Reconnection between Magnetic Clouds and Open Magnetic Fields: A Mechanism for the Erosion of Polar Coronal Holes?

We outline a method to determine the direction of solar open flux transport that results from the opening of magnetic clouds (MCs) by interchange reconnection at the Sun based solely on in-situ observations. This method uses established findings about i) the locations and magnetic polarities of emerging MC footpoints, ii) the hemispheric dependence of the helicity of MCs, and iii) the occurrence of interchange reconnection at the Sun being signaled by uni-directional suprathermal electrons inside MCs. Combining those observational facts in a statistical analysis of MCs during solar cycle 23 (period 1995 – 2007), we show that the time of disappearance of the northern polar coronal hole (1998 – 1999), permeated by an outward-pointing magnetic field, is associated with a peak in the number of MCs originating from the northern hemisphere and connected to the Sun by outward-pointing magnetic field lines. A similar peak is observed in the number of MCs originating from the southern hemisphere and connected to the Sun by inward-pointing magnetic field lines. This pattern is interpreted as the result of interchange reconnection occurring between MCs and the open field lines of nearby polar coronal holes. This reconnection process closes down polar coronal hole open field lines and transports these open field lines equatorward, thus contributing to the global coronal magnetic field reversal process. These results will be further constrainable with the rising phase of solar cycle 24.     

Empirical Modeling of Radiative <Emphasis Type="Italic">versus</Emphasis> Magnetic Flux for the Sun-as-a-Star

We study the relationship between full-disk solar radiative flux at different wavelengths and average solar photospheric magnetic-flux density, using daily measurements from the Kitt Peak magnetograph and other instruments extending over one or more solar cycles. We use two different statistical methods to determine the underlying nature of these flux – flux relationships. First, we use statistical correlation and regression analysis and show that the relationships are not monotonic for total solar irradiance and for continuum radiation from the photosphere, but are approximately linear for chromospheric and coronal radiation. Second, we use signal theory to examine the flux – flux relationships for a temporal component. We find that a well-defined temporal component exists and accounts for some of the variance in the data. This temporal component arises because active regions with high magnetic-field strength evolve, breaking up into small-scale magnetic elements with low field strength, and radiative and magnetic fluxes are sensitive to different active-region components. We generate empirical models that relate radiative flux to magnetic flux, allowing us to predict spectral-irradiance variations from observations of disk-averaged magnetic-flux density. In most cases, the model reconstructions can account for 85 – 90% of the variability of the radiative flux from the chromosphere and corona. Our results are important for understanding the relationship between magnetic and radiative measures of solar and stellar variability.     

A Comparison of Feature Classification Methods for Modeling Solar Irradiance Variation

Physical understanding of total and spectral solar irradiance variation depends upon establishing a connection between the temporal variability of spatially resolved solar structures and spacecraft observations of irradiance. One difficulty in comparing models derived from different data sets is that the many ways for identifying solar features such as faculae, sunspots, quiet Sun, and various types of “network” are not necessarily consistent. To learn more about classification differences and how they affect irradiance models, feature “masks” are compared as derived from five current methods: multidimensional histogram analysis of NASA/National Solar Observatory/Kitt Peak spectromagnetograph data, statistical pattern recognition applied to SOHO/Michelson Doppler Imager photograms and magnetograms, threshold masks allowing for influence of spatial surroundings applied to NSO magnetograms, and “one-trigger” and “three-trigger” algorithms applied to California State University at Northridge Cartesian Full Disk Telescope intensity observations. In general all of the methods point to the same areas of the Sun for labeling sunspots and active-region faculae, and available time series of area measurements from the methods correlate well with each other and with solar irradiance. However, some methods include larger label sets, and there are important differences in detail, with measurements of sunspot area differing by as much as a factor of two. The methods differ substantially regarding inclusion of fine spatial scale in the feature definitions. The implications of these differences for modeling solar irradiance variation are discussed. K.L. Harvey and S.R. Walton are deseased, to whom this paper is dedicated.     

Relationship between the Parameters of Coronal Holes and High-Speed Solar Wind Streams over an Activity Cycle

The comparison of the brightness and area of coronal holes (CH) to the solar wind speed, which was started by Obridko et al. (Solar Phys. 260, 191, 2009a) has been continued. While the previous work was dealing with a relatively short time interval 2000 – 2006, here we have analyzed the data on coronal holes observed in the Sun throughout activity Cycle 23. A catalog of equatorial coronal holes has been compiled, and their brightness and area variations during the cycle have been analyzed. It is shown that CH is not merely an undisturbed zone between the active regions. The corona heating mechanism in CH seems to be essentially the same as in the regions of higher activity. The reduced brightness is the result of a specific structure with the magnetic field being quasi-radial at as low an altitude as 1.1R _⊙ or a bit higher. The plasma outflow decreases the measure of emission from CH. With an adequate choice of the photometric boundaries, the CH area and brightness indices display a fairly high correlation (0.6 – 0.8) with the solar wind velocity throughout the cycle, except for two years, which deviate dramatically – 2001 and 2007, i.e., the maximum and the minimum of the cycle. The mean brightness of the darkest part of CH, where the field lines are nearly radial at low altitudes, is of the order of 18 – 20% of the solar brightness, while the brightness of the other parts of the CH is 30 – 40%. The solar wind streams originate at the base of the coronal hole, which acts as an ejecting nozzle. The solar wind parameters in CH are determined at the level where the field lines are radial.     

Comparisons of Supergranule Characteristics During the Solar Minima of Cycles 22/23 and 23/24

Supergranulation is a component of solar convection that manifests itself on the photosphere as a cellular network of around 35 Mm across, with a turnover lifetime of 1 – 2 days. It is strongly linked to the structure of the magnetic field. The horizontal, divergent flows within supergranule cells carry local field lines to the cell boundaries, while the rotational properties of supergranule upflows may contribute to the restoration of the poloidal field as part of the dynamo mechanism, which controls the solar cycle. The solar minimum at the transition from cycle 23 to 24 was notable for its low level of activity and its extended length. It is of interest to study whether the convective phenomena that influence the solar magnetic field during this time differed in character from periods of previous minima. This study investigates three characteristics (velocity components, sizes and lifetimes) of solar supergranulation. Comparisons of these characteristics are made between the minima of cycles 22/23 and 23/24 using MDI Doppler data from 1996 and 2008, respectively. It is found that whereas the lifetimes are equal during both epochs (around 18 h), the sizes are larger in 1996 (35.9 ± 0.3 Mm) than in 2008 (35.0 ± 0.3 Mm), while the dominant horizontal velocity flows are weaker (139 ± 1 m s⁻¹ in 1996; 141 ± 1 m s⁻¹ in 2008). Although numerical differences are seen, they are not conclusive proof of the most recent minimum being inherently unusual.     

Radio synthesis imaging of anisotropic angular broadening in the solar wind

We present Very Large Array observations at wavelengths of 2, 3.5, 6, and 20 cm, of angular broadening of radio sources due to the solar wind in the region 2–16 solar radii. Angular broadening is anisotropic with axial ratios in the range 2–16. Larger axial ratios are observed preferentially at smaller solar distances. Assuming that anisotropy is due to scattering blobs elongated along magnetic field lines, the distribution of position angles of the elliptically broadened images indicates that the field lines are non-radial even at the largest heliocentric distances observed here. At 5R⊙, the major axis scattering angle is ∼ 0.7" atλ= 6 cm and it varies with heliocentric distance asR ^-1.6. The level of turbulence, characterized by the wave structure function at a scale of 10 km along the major axis, normalized toλ = 20 cm, has a value 20 ± 7 at 5R⊙and varies with heliocentric distance asR ^-3. Comparison with earlier resu lts suggest that the level of turbulence is higher during solar maximum. Assuming a power-law spectrum of electron density fluctuations, the fitted spectral exponents have values in the range 2.8–3.4 for scale sizes between 2–35 km. The data suggests temporal fluctuations (of up to 10%) in the spectral exponent on a time scale of a few tens of minutes. The observed structure functions at different solar distances do not show any evidence for an inner scale; the upper limits are l k m at 2R⊙ and 4 km at 13R⊙. These upper limits are in conflict with earlier determinations and may suggest a reduced inner scale during solar maximum.     

Minor ions in the solar wind

Ions heavier than ⁴He are treated as “minors” in the solar wind. This is justified for many applications since minor ions have no significant influence on the dynamics of the interplanetary plasma. However, minor ions carry information on many aspects of the formation, on the acceleration and on the transfer of solar plasma from the corona into the interplanetary space. This review concentrates on various aspects of minor ions as diagnostic tracers. The elemental abundance patterns of the solar wind are shaped in the chromosphere and in the lower transition region by processes, which are not fully understood at this moment. Despite this lack of detailed understanding, observed abundance patterns have been classified and are now commonly used to characterize the sources, and to trace back solar-wind flows to their origins in the solar atmosphere. Furthermore, the solar wind is the most important source of information for solar isotopic abundances and for solar abundances of volatile elements. In order to fully exploit this information, a comprehensive understanding of elemental and isotopic fractionation processes is required. We provide observational clues to distinguish different processes at work.     

Turbulence In The Solar Atmosphere: Manifestations And Diagnostics Via Solar Image Processing

Intermittent magnetohydrodynamical turbulence is most likely at work in the magnetized solar atmosphere. As a result, an array of scaling and multi-scaling image-processing techniques can be used to measure the expected self-organization of solar magnetic fields. While these techniques advance our understanding of the physical system at work, it is unclear whether they can be used to predict solar eruptions, thus obtaining a practical significance for space weather. We address part of this problem by focusing on solar active regions and by investigating the usefulness of scaling and multi-scaling image-processing techniques in solar flare prediction. Since solar flares exhibit spatial and temporal intermittency, we suggest that they are the products of instabilities subject to a critical threshold in a turbulent magnetic configuration. The identification of this threshold in scaling and multi-scaling spectra would then contribute meaningfully to the prediction of solar flares. We find that the fractal dimension of solar magnetic fields and their multi-fractal spectrum of generalized correlation dimensions do not have significant predictive ability. The respective multi-fractal structure functions and their inertial-range scaling exponents, however, probably provide some statistical distinguishing features between flaring and non-flaring active regions. More importantly, the temporal evolution of the above scaling exponents in flaring active regions probably shows a distinct behavior starting a few hours prior to a flare and therefore this temporal behavior may be practically useful in flare prediction. The results of this study need to be validated by more comprehensive works over a large number of solar active regions. Sufficient statistics may also establish critical thresholds in the values of the multi-fractal structure functions and/or their scaling exponents above which a flare may be predicted with a high level of confidence. Based on the author's contributed talk “Manifestations and Diagnostics of Turbulence in the Solar Atmosphere”, presented at the Solar Image Processing Workshop II, Annapolis, Maryland, USA, 3–5 November 2004.     

The solar photospheric abundance of zirconium

Zirconium (Zr), together with strontium and yttrium, is an important element in the understanding of the Galactic nucleosynthesis. In fact, the triad Sr‐Y‐Zr constitutes the first peak of s‐process elements. Despite its general relevance not many studies of the solar abundance of Zr were conducted. We derive the zirconium abundance in the solar photosphere with the same CO⁵BOLD hydrodynamical model of the solar atmosphere that we previously used to investigate the abundances of C‐N‐O. We review the zirconium lines available in the observed solar spectra and select a sample of lines to determine the zirconium abundance, considering lines of neutral and singly ionised zirconium. We apply different line profile fitting strategies for a reliable analysis of Zr lines that are blended by lines of other elements. The abundance obtained from lines of neutral zirconium is very uncertain because these lines are commonly blended and weak in the solar spectrum. However, we believe that some lines of ionised zirconium are reliable abundance indicators. Restricting the set to Zr II lines, from the CO⁵BOLD 3D model atmosphere we derive A (Zr) = 2.62 ± 0.06, where the quoted error is the RMS line‐to‐line scatter (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Solar Chemical Abundances Determined with a CO5BOLD 3D Model Atmosphere

In the last decade, the photospheric solar metallicity as determined from spectroscopy experienced a remarkable downward revision. Part of this effect can be attributed to an improvement of atomic data and the inclusion of NLTE computations, but also the use of hydrodynamical model atmospheres seemed to play a role. This “decrease” with time of the metallicity of the solar photosphere increased the disagreement with the results from helioseismology. With a CO ⁵ BOLD 3D model of the solar atmosphere, the CIFIST team at the Paris Observatory re-determined the photospheric solar abundances of several elements, among them C, N, and O. The spectroscopic abundances are obtained by fitting the equivalent width and/or the profile of observed spectral lines with synthetic spectra computed from the 3D model atmosphere. We conclude that the effects of granular fluctuations depend on the characteristics of the individual lines, but are found to be relevant only in a few particular cases. 3D effects are not responsible for the systematic lowering of the solar abundances in recent years. The solar metallicity resulting from this analysis is Z=0.0153, Z/X=0.0209.     

Formation of Zr I and II lines under non-LTE conditions of stellar atmospheres

The formation of Zr I and Zr II lines in stellar atmospheres under non-LTE conditions has been considered for the first time. A model zirconium atom has been composed using 148 Zr I levels, 772 Zr II levels, and the ground Zr III state. Non-LTE calculations have been performed for model atmospheres with T _eff = 5500 and 6000 K, log g = 2.0 and 4.0, [M/H] = −3, −2, −1, 0. In the entire investigated range of parameters, the Zr I levels are shown to be underpopulated relative to their LTE populations in the line formation region. In contrast, the excited Zr II levels are overpopulated, while the ground state and lower excited levels of Zr II retain their LTE populations. Since the non-LTE effects cause the Zr I and Zr II spectral lines being investigated to weaken, the non-LTE corrections to the abundance derived from Zr I and Zr II lines are positive. For Zr II lines, they increase with decreasing metallicity and surface gravity up to 0.34 dex for the model with T _eff = 5500, log g = 2.0, and [M/H] = −2. The non-LTE effects depend weakly on temperature. The non-LTE corrections for Zr I lines reach 0.33 dex for solar-metallicity models. Zr I and Zr II lines in the solar spectrum have been analyzed. The non-LTE zirconium abundances derived from lines in the two ionization stages are shown to agree between themselves within the error limits, while the LTE abundance difference is 0.28 dex. The zirconium abundance in the solar atmosphere (averaged over Zr I and Zr II lines) is log ɛ_Zr,⊙ = 2.63 ± 0.07.     

The High-Resolution Solar Reference Spectrum between 250 and 550 nm and its Application to Measurements with the Ozone Monitoring Instrument

We have constructed a new high resolution solar reference spectrum in the spectral range between 250 and 550 nm. The primary use of this spectrum is for the calibration of the Dutch – Finnish Ozone Monitoring Instrument (OMI), but other applications are mentioned. The incentive for deriving a new high resolution solar reference spectrum is that available spectra do not meet our requirements on radiometric accuracy or spectral resolution. In this paper we explain the steps involved in constructing the new spectrum, based on available low and high resolution spectra and discuss the main sources of uncertainty. We compare the result with solar measurements obtained with the OMI as well as with other UV-VIS space-borne spectrometers with a similar spectral resolution. We obtain excellent agreement with the OMI measurements, which indicates that both the newly derived solar reference spectrum and our characterization of the OMI instrument are well understood. We also find good agreement with previously published low resolution spectra. The absolute intensity scale, wavelength calibration and representation of the strength of the Fraunhofer lines have been investigated and optimized to obtain the resulting high resolution solar reference spectrum.     

The Influence of Fluctuations of the Solar Emission Line Profileon the Doppler Shift of Interplanetary HLyα Lines Observed by the Hubble–Space–Telescope

In an earlier research the employment of a radiation transport model with angle-dependent partial frequency redistribution, self-absorption by interplanetary hydrogen, realistic solar H_Lyαemission profile, and a time dependent `hot' hydrogen model to analyze 5 interplanetary H_Lyα glow spectra obtained with theHubble–Space–Telescope–GHRS spectrometer, has not resulted in unequivocal determination of a set of thermodynamical parameters of the interstellar hydrogen The residual discrepancies between the model and the data concern the observations performed within an interval of 1 year close to the solar minimum from very similar lines of sight. In this paper we investigate by calculating interplanetary H_Lyα lines with the use of a one hydrogen distribution and several solar H_Lyα line profiles whether this residual may be caused by possible variations in time of the shape of the solar H_Lyα emission line profile which cause variable illuminations of the interplanetary gas. These variations of illuminations cause variations in Doppler shift of the resonant interplanetary H_Lyα line that can amount to ≃ 4 km s^-1in the line peak. Consequently we conclude that without adequate knowledge of the solar H_Lyα emission line profile during spectral observations of the interplanetary hydrogen gas it is impossible to obtain an agreement between models and observations better than by this value. This revised version was published online in July 2006 with corrections to the Cover Date.     

Crossing Filaments

Solar filaments show the position of large-scale polarity-inversion lines and are used for the reconstruction of large-scale solar magnetic field structure on the basis of Hα synoptic charts for the periods that magnetographic measurements are not available. Sometimes crossing filaments are seen in Hα filtergrams. We analyze daily Hα filtergrams from the archive of Big Bear Solar Observatory for the period of 1999 – 2003 to find crossing and interacting filaments. A number of examples are presented and filament patterns are compared with photospheric magnetic field distributions. We have found that all crossing filaments reveal quadrupolar magnetic configurations of the photospheric field and presume the presence of null points in the corona.     

The Temperature of the Low Corona During Solar Cycles 21–23

Observations of the forbidden coronal lines Fe xiv 530.3 nm and Fe x 637.4 nm obtained at the National Solar Observatory at Sacramento Peak are used to determine the variation of coronal temperature at latitudes above 30^∘ during solar activity cycles 21–23. Features of the long-term variation of emission in the two lines are also discussed. Temperatures at latitudes below 30^∘ are not studied because the technique used to determine the coronal temperature is not applicable in active regions. The polar temperature varies cyclically from approximately 1.3 to 1.7 MK. The temperatures are similar in both hemispheres. The temperature near solar minimum decreases strongly from mid-latitudes to the poles. The temperature of the corona above 80^∘ latitude generally follows the sunspot cycle, with minima in 1985 and 1995–1996 (cf. 1986 and 1996 for the smoothed sunspot number, Rz) and maxima in 1989 and 2000 (cf. 1989 and 2000 for Rz). The temperature of the corona above 30^∘ latitude at solar maximum is nearly uniform, i.e., there is little latitude dependence. If the maximum temperatures of cycles 22 and 23 are aligned in time (superposed epochs), the average annual N + S temperature (average of the northern and southern hemisphere) in cycle 23 is hotter than that in cycle 22 at all times both above 80^∘ latitude and above 30^∘ latitude. The difference in the average annual N + S maximum temperature between cycles 23 and 22 was 56 kK near the poles and 64 kK for all latitudes above 30^∘. Cycle 23 was also hotter at mid-latitudes than cycle 22 by 60 kK. The last 3 years of cycle 21 were hotter than the last 3 years of cycle 22. The difference in average annual N + S temperatures at the end of cycles 21 and 22 was 32 kK near the poles and 23 kK for all latitudes above 30^∘. Cycle 21 was also hotter at mid-latitudes than cycle 22 by at least 90 kK. Thus, there does not seem to be a solar-cycle trend in the low-coronal temperature outside of active regions.     

Periodic Appearance of Coronal Holes and the Related Variation of Solar Wind Parameters

We compared the variability of coronal hole (CH) areas (determined from daily GOES/SXI images) with solar wind (daily ACE data) and geomagnetic parameters for the time span 25 January 2005 until 11 September 2005 (late declining phase of solar cycle 23). Applying wavelet spectral analysis, a clear 9-day period is found in the CH time series. The GOES/SXI image sequence suggests that this periodic variation is caused by a mutual triangular distribution of CHs ∼120° apart in longitude. From solar wind parameters a 9-day periodicity was obtained as well, simultaneously with the 9-day period in the CH area time series. These findings provide strong evidence that the 9-day period in solar wind parameters, showing up as higher harmonic of the solar rotation frequency, is caused by the “periodic” longitudinal distribution of CHs on the Sun recurring for several solar rotations. The shape of the wavelet spectrum from the Dst index matches only weakly with that from the CH areas and is more similar to the wavelet spectrum of the solar wind magnetic field magnitude. The distinct 9-day period does not show up in sunspot group areas which gives further evidence that the solar wind modulation is strongly related to CH areas but not to active region complexes. The wavelet power spectra for the whole ACE data range (∼1998 – 2006) suggest that the 9-day period is not a singular phenomenon occurring only during a specific time range close to solar minimum but is occasionally also present during the maximum and decay phase of solar cycle 23. The main periods correspond to the solar rotation (27^d) as well as to the second (13.5^d) and third (9^d) harmonic. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

Cluster Observations of the Magnetospheric Low-Latitude Boundary Layer and Cusp during Extreme Solar Wind and Interplanetary Magnetic Field Conditions: II. 7 November 2004 ICME and Statistical Survey

We present a study of the plasma properties inside and dynamics of the low-latitude boundary layer (LLBL)/cusp during the ICME event on 7 November 2004 based on data from the four Cluster spacecraft. The interplanetary magnetic field (IMF) is predominantly strongly northward, up to 50 nT, with some short-duration rotations. The observed LLBL/cusp is very thick (∼6 – 7° invariant latitude (ILAT)) and migrates equatorward with rates of 0.55° and 0.04° ILAT per minute during quick southward IMF rotations and stable northward IMF, respectively. The LLBL/cusp observed by Cluster 1 and Cluster 4 is in a fast transition between different states and is populated by different types of plasma injection, presumably coming from multiple reconnection sites. During a period of extremely northward IMF, signatures of pulsed dual reconnection inside the LLBL/cusp are observed by Cluster 3, suggesting that at least part of the LLBL/cusp is on closed field lines. However, analysis of the ion data implies that the boundary layer is formed in the dawn sector of the magnetosphere and does not slowly convect from the dayside as has been suggested previously. A statistical study of the location of the LLBL/cusp equatorward boundary during the ICME events on 28 – 29 October 2003 and 7 – 10 November 2004 is performed. During extreme conditions the LLBL/cusp position is offset by −7° ILAT from the location under normal conditions, which might be explained by the influence of the high solar wind dynamic pressure. The LLBL/cusp moves equatorward with increasing southward and northward IMF. However, the LLBL/cusp position under strong southward IMF is more poleward than expected from previous studies, which could indicate some saturation in the dayside reconnection process or enhancement of the nightside reconnection rate. The LLBL/cusp position under strong northward IMF is extremely low and does not agree with the location predicted in previous studies. For the events with solar wind dynamic pressure >10 nPa, the LLBL/cusp position does not depend on the solar wind dynamic pressure. This might indicate some saturation in the mechanism of how the LLBL/cusp location depends on the solar wind dynamic pressure.