Active solar radio regions at metric frequencies and the interplanetary sector structures

The possible relation between type I noise active regions and the polarity distribution of the interplanetary magnetic field is examined for the period from 13 March to 21 August, 1968 (Solar Rotation Numbers 1842–1847) by using data from ground-based and satellite observations. In general four type I radio regions appeared during each solar rotation period except for Rotation No. 1842. The number of type I regions is the same as the number of sector boundaries. This result suggests that the configuration of the photospheric magnetic field extending into the interplanetary space may be related to the origin of the type I radio regions. Statistically the passage of the sector boundaries is delayed by approximately 5 days after the central meridian passage of the type I noise regions on the solar disk.The position of the source of the sector boundaries and its relation to the type I radio regions are investigated by taking into account the mean bulk velocity of solar winds as observed by space probes. A model of the large-scale structure of type I radio regions and their relation to the sector structure of the magnetic field as observed in the interplanetary space is briefly discussed.NASA Research Associate at the University of Maryland.     

Comparison of the mean photospheric magnetic field and the interplanetary magnetic field

The mean photospheric magnetic field of the sun seen as a star has been compared with the interplanetary magnetic field observed with spacecraft near the earth. Each change in polarity of the mean solar field is followed about 4 1/2 days later by a change in polarity of the interplanetary field (sector boundary). The scaling of the field magnitude from sun to near earth is within a factor of two of the theoretical value, indicating that large areas on the sun have the same predominant polarity as that of the interplanetary sector pattern. An independent determination of the zero level of the solar magnetograph has yielded a value of 0.1±0.05 G. An effect attributed to a delay of approximately one solar rotation between the appearance of a new photospheric magnetic feature and the resulting change in the interplanetary field is observed.     

Magnetospheric electric field from low latitude whistlers during magnetic storm

An attempt has been made to estimate the east-west component (E_w) of the magnetospheric equatorial electric field near L = 1.12 during a magnetic storm period from the whistlers observed at our low latitude ground station, Nainital (geomag.lat. 19°1'N), on March 25, 1971 in the 0130–0500 IST sector. The method of measuring E_w from the observed cross L-motions of whistler ducts within the plasmasphere, indicated by changes in nose frequency of whistlers, has been outlined. The nose frequencies of non-nose whistlers under consideration have been deduced from Dowden-Allcock linear Q-technique. The variation of ($\text{?}{}_{\mathrm{n}}\text{)}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ with local time has been shown, the slope of which can be directly related to the convection electric field. The estimated equatorial electric field at L? 1.12 is in the range 0.1–0.5 mV m^?1 (in the 0130–0500 IST sector) during a storm period, which is in agreement with the results reported by earlier workers. The departure from a dipole field and the contribution of an induced electric field from the temporal changes have been discussed. The importance of an electric field study has been indicated.     

The heliographic latitude dependence and sector structure of the interplanetary magnetic field 1969–1974: Results from the HEOS satellites

Data from the two HEOS satellites obtained during the period December 1968 to August 1974 are used to investigate the large-scale properties of the interplanetary magnetic field.The sector structure has been deduced from the observed times of sector boundary crossings which are tabulated. A two-sector pattern existed throughout most of the period with occasional intervals of 2–3 months duration in which four sectors appeared. The variation of the dominant sector polarity with heliographic latitude showed a reversal in sense during 1971 at the time of the reported reversal in the Sun's polar field. A statistical analysis of the change in polarity distribution with latitude suggests that at Earth's orbit the sector boundaries are inclined to the solar equator on average at an angle of 12 deg.No evidence was found in the HEOS measurements of the north-south field component to confirm the systematic latitude-dependent deviation of the plasma flow away from the solar equatorial plane suggested by several analyses of data from previous spacecraft. The mean field magnitude and the average amplitude of the directional fluctuations appeared to be independent of heliographic latitude within the ±7.3° range explored.     

Interplanetary magnetic clouds and cosmic ray modulation

We studied the cosmic ray intensity variation due to interplanetary magnetic clouds during an unusual class of low amplitude anisotropic wave train events. The low amplitude anisotropic wave train events in cosmic ray intensity have been identified using the data of ground based Deep River neutron monitor and studied during the period 1981–1994. Even though the occurrence of low amplitude anisotropic wave trains does not depend on the onset of interplanetary magnetic clouds, but the possibility of occurrence of these events cannot be overlooked during the periods of the interplanetary magnetic cloud events. It is observed that the solar wind velocity remains higher (> 300) than normal and the interplanetary magnetic field B remains lower than normal on the onset of the interplanetary magnetic cloud during the passage of low amplitude wave trains. It is also noted that the proton density remains significantly low during high solar wind velocity, which is expected. The north south component of interplanetary magnetic field Bz turns southward to one day before the arrival of cloud and remains in the southward direction after the arrival of a cloud. During these events the cosmic ray intensity is found to increase with increase of solar wind velocity. The superposed epoch analysis of cosmic ray intensity for these events during the onset of interplanetary magnetic clouds reveals that the decrease in cosmic ray intensity starts not at the onset of the cloud but after a few days. The cosmic ray intensity increases on arrival of the magnetic cloud and decreases gradually after the passage of the magnetic cloud.     

Relations between turbulent regions of interplanetary magnetic field and Jovian decametric radio wave emissions from the main source

Jovian decametric radio wave emissions that were observed at Goddard Space Flight Center, U.S.A. for a period from 1 October to 31 December, 1974 and data obtained at Mt Zao observatory, Tohoku University, Japan, for a period from 14 July to 6 December, 1975 have been used to investigate the relationship of the occurrence of the Jovian decametric radio waves (JDW), from the main source, to the geomagnetic disturbance index, ΣK_p. The dynamic cross-correlation between JDW and ΣK_p indicates an enhanced correlation for certain values of delay time. The delay time is consistent with predicted values based on a model of rotating turbulent regions in interplanetary space associated with two sector boundaries of the interplanetary magnetic field, i.e. the rotating sector boundaries of the interplanetary magnetic field first encounter the Earth's magnetosphere producing the geomagnetic field disturbances, and after a certain period, they encounter the Jovian magnetosphere. There are also cases where the order of the encounter is opposite, i.e. the sector boundaries encounter first Jovian magnetosphere and encounter the Earth's magnetosphere after a certain period.     

Dependence of inferred magnetic sector structure upon geomagnetic and solar activity

Inferred solar sector polarity given by the AC index of Svalgaard, has been intensively studied as a single time series and as a time series correlated with geomagnetic and solar activity. Power auto-spectra of the AC index yield a highly significant harmonic series with fundamental at 27 days period and possessing clear harmonics up to the sixth; and a very prominent peak at a period of 1 yr. The 27 day harmonic series clearly indicates the solar control of the index while the 1-yr period might be taken as confirmation of the work of Rosenberg and Coleman to the effect that the sector pattern observed on Earth depends upon Earth's heliographic latitude which has a 1-yr period.Cross correlation analysis and superposed epoch analysis are used to show that sectors inferred to be positive or away are associated with low geomagnetic and solar activity whereas sectors inferred to be negative or toward exhibit significantly enhanced geomagnetic and solar activity.These results appear to be in conflict with superposed epoch analyses by Wilcox and Ness using satellite observed sector polarities which showed that geomagnetic activity increased after passage of a sector boundary, independent of the nature, whether + ? or ? + of the boundary.The conflict is resolved here by noting that the yearly correlation coefficient, at zero time lag, between inferred sector structure and geomagnetic activity averaged about 0·5 for the year 1927–1958, dropped to low values by 1960, recovered by 1962 and then dropped sharply in 1963 by an order to magnitude; the correlation has remained essentially zero ever since. Thus, the satellite results, all obtained post 1963, would not show increased activity during either sector sign.The results cast doubt upon the accuracy of the early ‘inferred’ sector polarities because it is felt that the only simple explanation for the strange behavior of the correlation coefficient lies in some artifact of the data.     

Study of the Cosmic Ray Diurnal Anisotropy During Different Solar and Magnetic Conditions

The pressure-corrected hourly counting rate data of four neutron monitor stations have been employed to study the variation of cosmic ray diurnal anisotropy for a period of about 50 years (1955–2003). These neutron monitors, at Oulu ( R _c = 0.78 GV), Deep River ( R _c = 1.07 GV), Climax ( R _c = 2.99 GV), and Huancayo ( R _c = 12.91 GV) are well distributed on the earth over different latitudes and their data have been analyzed. The amplitude of the diurnal anisotropy varies with a period of one solar cycle (∼11 years), while the phase varies with a period of two solar cycles (∼22 years). In addition to its variation on year-to-year basis, the average diurnal amplitude and phase has also been calculated by grouping the days for each solar cycle, viz. 19, 20, 21, 22, and 23. As a result of these groupings over solar cycles, no significant change in the diurnal vectors (amplitude as well as phase) from one cycle to other has been observed. Data were analyzed by arranging them into groups on the basis of the polarity of the solar polar magnetic field and consequently on the basis of polarity states of the heliosphere ( A > 0 and A < 0). Difference in time of maximum of diurnal anisotropy (shift to earlier hours) is observed during A < 0 (1970s, 1990s) polarity states as compared to anisotropy observed during A > 0 (1960s, 1980s). This shift in phase of diurnal anisotropy appears to be related to change in preferential entry of cosmic ray particles (via the helioequatorial plane or via solar poles) into the heliosphere due to switch of the heliosphere from one physical/magnetic state to another following the solar polar field reversal.     

Rotation cycles of the sector structure of the solar magnetic field and its activity

We study the rotation of the sector structure of the solar magnetic field by using Stanford magnetographic observations from 1975 until 2000 and magnetic synoptic Hα-maps obtained from 1904 until 2000. The two independent series of observations yielded the same rotation periods of the two-sector (26.86 days) and four-sector (13.64 days) structures. We introduce a new index of the solar rotation, SSPM(t). The spectral power density of the sector structure of the magnetic field is shown to exhibit a 22-year cyclicity. The two-and four-sector structures of the magnetic field rotate faster at the maxima of even 11-year sunspot cycles. This phenomenon may be called the Gnevyshev-Ohl rule for the solar rotation. The 11-year sector-structure activity cycles are shown to lead the 11-year sunspot cycles (Wolf numbers) by 5.5 years. A 55-year component with the slowest rotation in the 18th cycle (1945–1955) was distinguished in the sector-structure rotation.     

Solar Rotational Periodicities and the Semiannual Variation in the Solar Wind, Radiation Belt, and Aurora

The behavior of a number of solar wind, radiation belt, auroral and geomagnetic parameters is examined during the recent extended solar minimum and previous solar cycles, covering the period from January 1972 to July 2010. This period includes most of the solar minimum between Cycles 23 and 24, which was more extended than recent solar minima, with historically low values of most of these parameters in 2009. Solar rotational periodicities from 5 to 27 days were found from daily averages over 81 days for the parameters. There were very strong 9-day periodicities in many variables in 2005?–?2008, triggered by recurring corotating high-speed streams (HSS). All rotational amplitudes were relatively large in the descending and early minimum phases of the solar cycle, when HSS are the predominant solar wind structures. There were minima in the amplitudes of all solar rotational periodicities near the end of each solar minimum, as well as at the start of the reversal of the solar magnetic field polarity at solar maximum (～?1980, ～?1990, and ～?2001) when the occurrence frequency of HSS is relatively low. Semiannual equinoctial periodicities, which were relatively strong in the 1995?–?1997 solar minimum, were found to be primarily the result of the changing amplitudes of the 13.5- and 27-day periodicities, where 13.5-day amplitudes were better correlated with heliospheric daily observations and 27-day amplitudes correlated better with Earth-based daily observations. The equinoctial rotational amplitudes of the Earth-based parameters were probably enhanced by a combination of the Russell-McPherron effect and a reduction in the solar wind-magnetosphere coupling efficiency during solstices. The rotational amplitudes were cross-correlated with each other, where the 27-day amplitudes showed some of the weakest cross-correlations. The rotational amplitudes of the >?2 MeV radiation belt electron number fluxes were progressively weaker from 27- to 5-day periods, showing that processes in the magnetosphere act as a low-pass filter between the solar wind and the radiation belt. The A _p/K _p magnetic currents observed at subauroral latitudes are sensitive to proton auroral precipitation, especially for 9-day and shorter periods, while the A _p/K _p currents are governed by electron auroral precipitation for 13.5- and 27-day periodicities.     

Nightglow (557.7 nm of OI) in the central polar cap

The 557.7 nm OI night airglow emission was measured in the central polar cap by ground-based photometric systems at Thule Air Base, Greenland during the winter seasons from 1972–1973 to 1974–1975 and at Thule-Qanaq, Greenland during the winter season of 1973–1974. The behavior of the 557.7 nm night airglow emission in the polar cap was found to be quite different from that observed at mid and low latitudes. No diurnal variation greater than ±5% exist in the data. Large amplitude variations in the 557.7 nm daily average emission intensities can change by up to a factor of approximately 8 over periods ranging from 4 to 19 days. These long-term airglow variations cover at least a 100 km horizontal range as determined by a correlation coefficient of 0.94 between daily average 557.7 nm airglow intensities observed at Thule Air Base and Thule-Qanaq. An interplanetary magnetic field sector related behavior is evident in the daily average intensities which shows an increase of intensity in a positive (+) sector and a decrease of intensity in a negative (?) sector. No significant correlation was found between the 557.7 nm daily average intensities and Zurich sunspot number R_Z, although a season to season positive trend was evident. Correlations between the 557.7 nm daily average intensities and planetary magnetic indices ΣK_p and A_p were found to be inconclusive due to sector related effects. The Barth and Chapman mechanisms are discussed as possible source mechanisms for the 557.7 nm airglow in the central polar cap, and a hypothesis is presented to explain the airglow variations.     

Temporal variations in the large-scale magnetic field of the solar atmosphere at heights from the photosphere to the source surface

Calculations of the magnetic field in the potential approximation (using Bd technology (Rudenko, 2001)) were used to study the time variations of several parameters of the large-scale magnetic field in the solar atmosphere during the last four cycles. Synoptic maps (SMs) for the radial component Br of the calculated magnetic field were plotted at 10 heights between the solar surface (R = R _⊙) and the source (R = 2.5R _⊙). On these SMs, we marked the 10-degree latitudinal areas. The following (averaged within the zone) characteristics of the magnetic field were determined corresponding to these zones: Sp, Sm; S _+fields , where Sp is the positive value of Br, Sm is the averaged modulus of the negative Br; S _+fields is the percentage of latitudinal zones with positive Br. The analysis of temporal variations in the magnitude of S points to different origins of the large-scale magnetic field in the near-equatorial and polar regions of the solar atmosphere. The analysis of temporal variations of S _+fields showed that there were almost no periods with a mixed polarity at R = 2.5R _⊙ during the 21st and 22nd solar cycles and in an ascending phase of the 23rd cycle. However, beginning from the maximum of the 23rd cycle, a mixed polarity in the equatorial region was observed until the end of the long minimum of activity. We hypothesized that this could be a precursor for a long minimum between the 23rd and 24th solar cycles. It was shown that during the maximum phase of the 24th solar cycle the magnetic field at R = R _⊙ is much less than that during the maximum phase of the 23rd cycle, and in the region from 55° to 75°, this difference reaches an order of magnitude.     

Flapping motions of the tail plasma sheet induced by the interplanetary magnetic field variations

Flapping motions of the magnetotail with an amplitude of several earth radii are studied by analysing the observations made in the near (x = ?25 ~ ?30 R_E and the distant (x? ?60 R_E) tail regions. It is found that the flapping motions result from fluctuations in the interplanetary magnetic field, especially Alfvénic fluctuations, when the magnitude of the interplanetary magnetic field is larger than ~10 γ and they propagate behind the Earth with the solar wind flow. Flappings tend to be observed in early phases of the magnetospheric substorm, and they have two fundamental modes with periods of ~200 and ~500 sec. In some limited cases a good correspondence with the long period micropulsations (Pc5) in the polar cap region is observed. These observational results are explained by the model in which the Alfvénic fluctuations in the solar wind penetrate into the magnetosphere along the connected interplanetary-magnetospheric field lines. The characteristics of the flapping reveal that the geomagnetic tail is a good resonator for the hydromagnetic disturbances in the solar wind.     

Dependence of the magnetopause position on the southward interplanetary magnetic field

The distance to the dayside magnetopause is statistically analyzed in order to detect the possible dependence of the dayside magnetic flux on the polarity of the interplanetary magnetic field. The effect of changing solar wind pressure is eliminated by normalizing the observed magnetopause distances by the simultaneous solar wind pressure data. It is confirmed that the normalized size of the dayside magnetosphere at the time of southward interplanetary magnetic field is smaller than that at the time of northward interplanetary magnetic field. The difference in the magnetopause position between the two interplanetary field polarity conditions ranges from 0 to 2R_E. Statistics of the relation between the magnetopause distance and the magnetic field intensity just inside the magnetopause testifies that the difference in the magnetopause position is not due to a difference in the magnetosheath plasma pressure. The effect of the southward interplanetary magnetic field is seen for all longitudes and latitudes investigated (|λ_GM|? 45°, |φ_SM|? 90°). These results strongly suggest that a part of the dayside magnetic flux is removed from the dayside at the time of southward interplanetary magnetic field.     

The mean coronal magnetic field determined from HELIOS Faraday rotation measurements

Coronal Faraday rotation of the linearly polarized carrier signals of the HELIOS spacecraft was recorded during the regularly occurring solar occultations over almost a complete solar cycle from 1975 to 1984. These measurements are used to determine the average strength and radial variation of the coronal magnetic field at solar minimum at solar distances from 3–10 solar radii, i.e., the range over which the complex fields at the coronal base are transformed into the interplanetary spiral. The mean coronal magnetic field in 1975–1976 was found to decrease with radial distance according to r ^?α, where α = 2.7 ± 0.2. The mean field magnitude was 1.0 ± 0.5 × 10 ^?5 tesla at a nominal solar distance of 5 solar radii. Possibly higher magnetic field strengths were indicated at solar maximum, but a lack of data prevented a statistical determination of the mean coronal field during this epoch.     

Effects of interplanetary magnetic field and magnetospheric substorm variations on the dayside aurora

Photometric observations of dayside auroras are compared with simultaneous measurements of geomagnetic disturbances from meridian chains of stations on the dayside and on the nightside to document the dynamics of dayside auroras in relation to local and global disturbances. These observations are related to measurements of the interplanetary magnetic field (IMF) from the satellites ISEE-1 and 3. It is shown that the dayside auroral zone shifts equatorward and poleward with the growth and decay of the circum-oval/polar cap geomagnetic disturbance and with negative and positive changes in the north-south component of the interplanetary magnetic field (B_z). The geomagnetic disturbance associated with the auroral shift is identified as the DP2 mode. In the post-noon sector the horizontal disturbance vector of the geomagnetic field changes from southward to northward with decreasing latitude, thereby changing sign near the center of the oval precipitation region. Discrete auroral forms are observed close to or equatorward of the ΔH = 0 line which separates positive and negative H-component deflections. This reversal moves in latitude with the aurora and it probably reflects a transition of the electric field direction at the polar cap boundary. Thus, the discrete auroral forms observed on the dayside are in the region of sunward-convecting field lines. A model is proposed to explain the equatorward and poleward movement of the dayside oval in terms of a dayside current system which is intensified by a southward movement of the IMF vector. According to this model, the Pedersen component of the ionospheric current is connected with the magnetopause boundary layer via field-aligned current (FAC) sheets. Enhanced current intensity, corresponding to southward auroral shift, is consistent with increased energy extraction from the solar wind. In this way the observed association of DP2 current system variations and auroral oval expansion/contraction is explained as an effect of a global, ‘direct’ response of the electromagnetic state of the magnetosphere due to the influence of the solar wind magnetic field. Estimates of electric field, current, and the rate of Joule heat dissipation in the polar cap ionosphere are obtained from the model.     

The magnetic field draping direction at Mars from April 1999 through August 2004

Using more than five years of data from the magnetometer and electron reflectometer (MAG/ER) on Mars Global Surveyor (MGS), we derive the draping direction of the magnetic field above a given latitude band in the northern hemisphere. The draping direction varies on timescales associated with the orbital period of Mars and with the solar rotation period. We find that there is a strongly preferred draping direction when Mars is in one solar wind sector, but the opposite direction is not preferred as strongly for the other solar wind sector. This asymmetry occurs at or below the magnetic pileup boundary (MPB), is observed preferentially on field lines that connect to the collisional ionosphere, and is independent of planetary longitude. The observations could be explained by a hemispherical asymmetry in the access of field lines to the low-altitude ionosphere, or possibly from global modification of the low-altitude solar wind interaction by crustal magnetic fields. We show that the draping direction affects both the penetration of sheath plasma to 400 km altitudes on the martian dayside and the radial component of the magnetic field on the planetary night side.     

Solar-interplanetary modeling: 3-D solar wind solutions in prescribed non-radial magnetic field geometries

A model is presented which describes the 3-dimensional non-radial solar wind expansion between the Sun and the Earth in a specified magnetic field configuration subject to synoptically observed plasma properties at the coronal base. In this paper, the field is taken to be potential in the inner corona based upon the Mt. Wilson magnetograph observations and radial beyond a certain chosen surface. For plasma boundary conditions at the Sun, we use deconvoluted density profiles obtained from synopticK-coronameter brightness observations. The temperature is taken to be 2 × 10⁶ K at the base of closed field lines and 1.6 x 10⁶K at the base of open field lines. For a sample calculation, we employ data taken during the period of the 12 November 1966 eclipse. Although qualitative agreement with observations at 1 AU is obtained, important discrepancies emerge which are not apparent from spherically symmetric models or those models which do not incorporate actual observations in the lower corona. These discrepancies appear to be due to two primary difficulties - the rapid geometric divergence of the open field lines in the inner corona as well as the breakdown in the validity of the Spitzer heat conduction formula even closer to the Sun than predicted by radial flow models. These two effects combine to produce conductively dominated solutions and lower velocities, densities, and field strengths at the Earth than those observed. The traditional difficulty in solar wind theory in that unrealistically small densities must be assumed at the coronal base in order to obtain observed densities at 1 AU is more than compensated for here by the rapid divergence of field lines in the inner corona. For these base conditions, the value ofβ(ratio of gas pressure to magnetic pressure) is shown to be significantly greater than one over most of the lower corona - suggesting that, for the coronal boundary conditions used here, the use of a potential or force-free magnetic field configuration may not be justified. The calculations of this paper point to the directions where future research on solar-interplanetary modelling should receive priority:

1. better models for the coronal magnetic field structure
2. improved understanding of the thermal conductivity relevant for the solar wind plasma.

     

Allowance for the geometry of solar wind interaction with the Earth’s magnetic field in geoeffective parameters and in the prediction of geomagnetic activity

We suggest geoeffective independent parameters that can be calculated on the basis of conventional measurements of the solar wind, which allows them to be used to forecast space weather. We present the results of our analysis of the ground variations in planetary geomagnetic activity (K _p ) and geoeffective parameters calculated on the basis of solar wind and interplanetary magnetic field measurements in the Earth’s orbit for the period 1964–1996 by taking into account the change in the orientation of the geomagnetic moment during the Earth’s diurnal and annual motions.     

Small-scale background magnetic field on the sun in solar cycle 23

Based on SOHO/MDI data (an archive of magnetic maps with a resolution of ～2″), we have investigated the dynamics of the small-scale background magnetic field on the Sun in solar cycle 23. The cyclic variations and surface structure of the background magnetic field have been analyzed using the mean estimates of 〈B〉 and 〈B ²〉 of the observed magnetic field strength B for various solar surface areas and at various B levels. We have established that the cyclic variations of 〈2〉 at latitudes below 30° are essentially similar to those of the total radio flux F _10.7. A significant difference between the background magnetic fields in the northern and southern solar hemispheres persisting throughout the solar cycle has been detected. We have found the effect of background magnetic field growth toward the solar limb and concluded that the transversal component in the background magnetic field is significant. The relatively weak small-scale background magnetic fields are shown to form a special population with its own special laws of cyclic variation.