Observations of the response time of high-latitude ionospheric convection to variations in the interplanetary magnetic field using EISCAT and IMP-8 data

We have combined ∼300 h of tristatic measurements of the field-perpendicular F region ionospheric flow measured overhead at Tromsø by the EISCAT UHF radar, with simultaneous IMP-8 measurements of the solar wind and interplanetary magnetic field (IMF) upstream of the Earth’s magnetosphere, in order to examine the response time of the ionospheric flow to changes in the north-south component of the IMF (B_z). In calculating the flow response delay, the time taken by field changes observed by the spacecraft to first effect the ionosphere has been carefully estimated and subtracted from the response time. Two analysis methods have been employed. In the first, the flow data were divided into 2 h-intervals of magnetic local time (MLT) and cross-correlated with the “half-wave rectifier” function V²B_s, where V is the solar wind speed, and B_s is equal to IMF B_z if the latter is negative, and is zero otherwise. Response delays, determined from the time lag of the peak value of the cross-correlation coefficient, were computed versus MLT for both the east-west and north-south components of flow. The combined data set suggests minimum delays at ∼1400 MLT, with increased response times on the nightside. For the 12-h sector centred on 1400 MLT, the weighted average response delay was found to be 1.3 ± 0.8 min, while for the 12-h sector centred on 0200 MLT the weighted average delay was found to increase to 8.8 ± 1.7 min. In the second method we first inspected the IMF data for sharp and enduring (at least ∼5 min) changes in polarity of the north-south component, and then examined concurrent EISCAT flow data to determine the onset time of the corresponding enhancement or decay of the flow. For the case in which the flow response was timed from whichever of the flow components responded first, minimum response delays were again found at ∼1400 MLT, with average delays of 4.8 ± 0.5 min for the 12-h sector centred on 1400 MLT, increasing to 9.2 ± 0.8 min on the nightside. The response delay is thus found to be reasonably small at all local times, but typically ∼6 min longer on the nightside compared with the dayside. In order to make an estimate of the ionospheric information propagation speed implied by these results, we have fitted a simple theoretical curve to the delay data which assumes that information concerning the excitation and decay of flow propagates with constant speed away from some point on the equatorward edge of the dayside open-closed field line boundary, taken to lie at 77° magnetic latitude. For the combined cross-correlation results the best-fit epicentre of information propagation was found to be at 1400 MLT, with an information propagation phase speed of 9.0 km s⁻¹. For the combined event analysis, the best-fit epicentre was also found to be located at 1400 MLT, with a phase speed of 6.8 km s⁻¹.     

SuperDARN studies of the ionospheric convection response to a northward turning of the interplanetary magnetic field

The response of the dayside ionospheric flow to a sharp change in the direction of the interplanetary magnetic field (IMF) measured by the WIND spacecraft from negative B_z and positive B_y, to positive B_z and small B_y, has been studied using SuperDARN radar, DMSP satellite, and ground magnetometer data. In response to the IMF change, the flow underwent a transition from a distorted twin-cell flow involving antisunward flow over the polar cap, to a multi-cell flow involving a region of sunward flow at high latitudes near noon. The radar data have been studied at the highest time resolution available (2 min) to determine how this transition took place. It is found that the dayside flow responded promptly to the change in the IMF, with changes in radar and magnetic data starting within a few minutes of the estimated time at which the effects could first have reached the dayside ionosphere. The data also indicate that sunward flows appeared promptly at the start of the flow change (within 2 min), localised initially in a small region near noon at the equatorward edge of the radar backscatter band. Subsequently the region occupied by these flows expanded rapidly east-west and poleward, over intervals of 7 and 14 min respectively, to cover a region at least 2 h wide in local time and 5° in latitude, before rapid evolution ceased in the noon sector. In the lower latitude dusk sector the evolution extended for a further 6 min before quasi-steady conditions again prevailed within the field-of-view. Overall, these observations are shown to be in close conformity with expectations based on prior theoretical discussion, except for the very prompt appearance of sunward flows after the onset of the flow change.     

Trends in the F2 ionospheric layer due to long-term variations in the Earth's magnetic field

The Earth's magnetic field presents long-term variations with changes in strength and orientation. Particularly, changes in the dip angle (I) and, consequently, in the sin(I)cos(I) factor, affect the thermospheric neutral winds that move the conducting plasma of the ionosphere. In this way, a lowering or lifting of the F2-peak (hmF2) is induced together with changes in foF2, depending on season, time and location. A simple theoretical approximation, developed in a previous work, is extended to a worldwide latitude–longitude grid to assess hmF2 and foF2 trends due to Earth's magnetic field secular variations. Compared to the greenhouse gases effects over the ionosphere, the Earth's magnetic field may be able to produce stronger trends which vary with season, time and location. However, to elucidate the origin of F2-region trends, long-term variations in the three possible known mechanisms should be considered altogether—greenhouse gases, geomagnetic activity and Earth's magnetic field.     

Interhemispheric contrasts in the ionospheric convection response to changes in the interplanetary magnetic field and substorm activity: a case-study

Interhemispheric contrasts in the ionospheric convection response to variations of the interplanetary magnetic field (IMF) and substorm activity are examined, for an interval observed by the Polar Anglo-American Conjugate Experiment (PACE) radar system between 1600 and 2100 MLT on 4 March 1992. Representations of the ionospheric convection pattern associated with different orientations and magnitudes of the IMF and nightside driven enhancements of the auroral electrojet are employed to illustrate a possible explanation for the contrast in convection flow response observed in radar data at nominally conjugate points. Ion drift measurements from the Defence Meteorological Satellite Program (DMSP) confirm these ionospheric convection flows to be representative for the prevailing IMF orientation and magnitude. The location of the fields of view of the PACE radars with respect to these patterns suggest that the radar backscatter observed in each hemisphere is critically influenced by the position of the ionospheric convection reversal boundary (CRB) within the radar field of view and the influence it has on the generation of the irregularities required as scattering targets by high-frequency coherent radar systems. The position of the CRB in each hemisphere is strongly controlled by the relative magnitudes of the IMF B_z and B_y components, and hence so is the interhemispheric contrast in the radar observations.     

日面方位磁场扰动和行星际磁场螺旋结构

文采用球坐标下2.5维理想MHD模型,对日球子午面内方位磁场扰动的传播进行数值模拟,重点分析它对行星际磁场螺旋角的影响. 本文认为,观测到的行星际磁场螺旋角大于Parker模型的预言值,是太阳表面不断向行星际发出同向方位磁场扰动的结果;太阳较差自转在太阳内部产生的方位磁场为这类扰动提供了源头. 模拟结果表明,采用持续时间等于周期的十分之一、扰动幅度为103nT量级的正向方位磁场扰动,就可使1 AU处行星际磁场的螺旋角增加2°左右,与有关观测结果相符. 模拟结果还表明,上述方位磁场扰动对日球子午面内的太阳风特性和磁场位形的影响基本上可以忽略.     

Terrestrial, planetary and interplanetary magnetic fields

A discussion meeting at Burlington House on 10 March 2000 featured eight speakers and an audience of about 50. Andy Smith reports.     

Effect of magnetic storms in variations in the atmospheric electric field at midlatitudes

The observations of the variations in the vertical component of the atmospheric electric field (E _z ) at Swider midlatitude Poland observatory (geomagnetic latitude 47.8°) under the conditions of fair weather during 14 magnetic storms have been analyzed. The effect of the magnetic storm main phase in the daytime midlatitude variations in E _z in the absence of local geomagnetic disturbances has been detected for the first time. Considerable (～100–300 V m^?1) decreases in the electric field strength (E _z ) at Swider observatory were observed in daytime simultaneously with the substorm onset in the nighttime sector of auroral latitudes (College observatory). The detected effects indicate that an intensification of the interplanetary electric field during the magnetic storm main phase, the development of magnetospheric substorms, and precipitation of energetic electrons into the nighttime auroral ionosphere can result in considerable disturbances in the midlatitude atmospheric electric field.     

Midday reversal of equatorial ionospheric electric field

A comparative study of the geomagnetic and ionospheric data at equatorial and low-latitude stations in India over the 20 year period 1956–1975 is described. The reversal of the electric field in the ionosphere over the magnetic equator during the midday hours indicated by the disappearance of the equatorial sporadic E region echoes on the ionograms is a rare phenomenon occurring on about 1% of time. Most of these events are associated with geomagnetically active periods. By comparing the simultaneous geomagnetic H field at Kodaikanal and at Alibag during the geomagnetic storms it is shown that ring current decreases are observed at both stations. However, an additional westward electric field is superimposed in the ionosphere during the main phase of the storm which can be strong enough to temporarily reverse the normally eastward electric field in the dayside ionosphere. It is suggested that these electric fields associated with the V × Bz electric fields originate at the magnetopause due to the interaction of the solar wind and the interplanetary magnetic field.     

Trends and the sector structure of the interplanetary magnetic field

Large-scale variations in the interplanetary magnetic field (IMF) are studied using its measurements by the Advanced Composition Explorer (ACE) spacecraft. To reveal the sector structure, an algorithm for estimating trends of long time series is proposed. The algorithm makes it possible to determine the sectors as well as to trace the tendencies in changes in the “long-lived” IMF structures for various degrees of initial data smoothing.     

Effects of the interplanetary magnetic field sector structure in the winter ionosphere

Summary The ionospheric effects of the interplanetary magnetic field (IMF) sector boundary crossings are studied for the winters of 1963–69. They are considerably stronger for proton than for non-proton sector boundaries. There are two different types of effects. The geomagnetic type is a disturbance, observed in geomagnetic activity, the night-time ionosphere and the day-time F2 region near the geomagnetic equator. The effect in the ionosphere is interpreted in terms of the IMF sector boundary crossing related changes in geomagnetic activity. The tropospheric type is aquietening, observed in tropospheric vorticity and in the day-time mid-and low-latitude ionosphere (except the geomagnetic equator region). The mechanism of this effect remains unexplained.

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¶rt;m u m nu mau nam aum n () ¶rt; u 1963–69. u m u ¶rt; nm ¶rt; a mau. mm ¶rt;a m¶rt; muna m. aum m u, a¶rt;a aum amumu, u u ¶rt; F2 amu uuaum ama. mu u m ¶rt;mu uu aum amumu, m a nu mau . n mun m nu, a¶rt; aumu mn u ¶rt; ¶rt;- u uum u (a uu amuaum ama). au m ma um.

     

Daytime efficiency and characteristic time scale of interplanetary electric fields penetration to equatorial latitude ionosphere

We analyze the daytime efficiency of the interplanetary electric field (IEF) penetration to the equatorial ionosphere based on a correlation analysis carried out between different levels of decomposition applied to IEF intensity measured at the ACE spacecraft and ionospheric electric field intensity inferred from ground based magnetometers located in the equatorial region in Brazil. We compare the time variations of those two electric field intensities by means of a scale-by-scale decomposed time series through wavelet multi-resolution analysis. Efficiency is here defined as the fraction of the variation of the IEF intensity that has penetrated into the equatorial region, and it increases with increasing fraction. Two cases of prompt penetration electric fields (PPEF) are analyzed: one occurring on March 31, 2001, and other on April 17, 2002. Our results show that the penetration effect with time scale ranging around 1 h is maximized in relation to other scales.     

The interplanetary magnetic field sector structure and meteorological microseisms

Summary Meteorological microseisms, recorded at the Prague seismic station and, similarly, at other Central Europe stations, reflect certain meteorological situations in the North Atlantic frontal zone. Meteorological elements in this zone are affected by some factors of extra-terrestrial origin, among others by the interplanetary magnetic field (IMF) sector structure. The analysis of the 68 IMF sector boundary passage effects in microseismic activity, expressed in microseismic amplitudes, showed a well-developed minimum of microseismic activity on the day of the boundary passage in winter, but no such effect in spring and autumn. In spring and autumn, however, a tendency was observed to slightly higher microseismic activity in the away (+) sector, whereas no such tendency was observed in winter. These results agree with the IMF effect found in the troposphere. Generally, the IMF sector structure is only a modulating rather than a major factor controlling the microseismic activity.     

Daubechies wavelet coefficients: a tool to study interplanetary magnetic field fluctuations

We have studied a set of 41 magnetic clouds (MCs) measured by the ACE spacecraft, using the discrete orthogonal wavelet transform (Daubechies wavelet of order two) in three regions: Pre-MC (plasma sheath), MC and Post-MC. We have used data from the IMF GSM-components with time resolution of 16 s. The mathematical property chosen was the statistical mean of the wavelet coefficients (〈Dd1 〉). The Daubechies wavelet coefficients have been used because they represent the local regularity present in the signal being studied. The results reproduced the well-known fact that the dynamics of the sheath region is more than that of the MC region. This technique could be useful to help a specialist to find events boundaries when working with IMF datasets, i.e., a best form to visualize the data. The wavelet coefficients have the advantage of helping to find some shocks that are not easy to see in the IMF data by simple visual inspection. We can learn that fluctuations are not low in all MCs, in some cases waves can penetrate from the sheath to the MC. This methodology has not yet been tested to identify some specific fluctuation patterns at IMF for any other geoeffective interplanetary events, such as Co-rotating Interaction Regions (CIRs), Heliospheric Current Sheet (HCS) or ICMEs without MC signatures. In our opinion, as is the first time that this technique is applied to the IMF data with this purpose, the presentation of this approach for the Space Physics Community is one of the contributions of this work.     

The response of the azimuthal component of the ionospheric electric field to auroral arc brightening

We have analyzed the response of azimuthal component of the ionospheric electric field to auroral arc activity. We have chosen for analysis three intervals of coordinated EISCAT and TV observations on 18 February, 1993. These intervals include three kinds of arc activity: the appearance of a new auroral arc, the gradual brightening of the existing arc and variations of the arc luminosity. The arcs were mostly east-west aligned. In all cases, the enhancement of arc luminosity is accompanied by a decrease in the westward component of the ionospheric electric field. In contrast, an increase of that component seems to be connected with arc fading. The observed response is assumed to have the same nature as the short circuit of an external electric field by the conductor. The possible consequence of this phenomenon is discussed.     

Does the interplanetary magnetic field and the solar activity contribute to the tropospheric circulation?

A significant tendency is shown for both Etesians and (+, –) sector boundaries of the interplanetary magnetic field (IMF) to occur on the same solar rotation days, during the main period of the Etesians effect (July–August). In addition, the solar activity seems to control the Etesians distribution within the IMF sector structure. In the epoch of maximum there is a significant tendency of Etesians to occur during toward IMF days. In contrast, in the epoch of minimum Etesians occur mainly during away IMF days. Finally, in the epoch of intermediate the Etesians are uniformly distributed in away and toward IMF days. Since these conclusions are statistically significant at high confidence levels, it is fair to assume that IMF and solar activity seem to contribute, to some extent, to the Etesians occurrences, as well as to their distribution within the solar rotation and the IMF sector structure; that is, some solar contribution to the tropospheric circulation is implied.     

On the structure of the interplanetary magnetic field under non-stationary boundary conditions

Summary The structure of the interplanetary magnetic field (IMF) is investigated theoretically in a kinematic approximation under frozen-in conditions in the solar wind and non-stationary boundary conditions. As an example, a time-dependent model of the IMF, created in the case of a change of the general magnetic field of the Sun represented by the dipole term, is analyzed. Very simple assumptions as to the field of velocities in the solar wind are made. The results show the formation of zero IMF points of two types(O, X). Points of theO-type are formed and move radially in the equatorial plane. They are surrounded by magnetic clouds with loops of lines of force. Points of theX-type are formed and move radially above the poles.

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muu u¶rt;aa mma nam aum n() uamu nuuuu nu uu mu m u mauaau uu. am nua ama ¶rt; , ua nu uuu aum n a, n¶rt;ma ¶rt;un , auu m u. mum n m m ¶rt;a nmu n¶rt;nu. mam naam aau m ¶rt; mun(O, X). uO-muna uam u ¶rt;um a¶rt;ua amua nmu. u aumu aau nmu u uu. uX-muna uam u ¶rt;um a¶rt;ua a¶rt; nau.

     

On the connection between the atmospheric electric field measured at the surface and the ionospheric electric field in the Central Antarctica

Regular measurements of the atmospheric electric field made at Vostok Station (φ=78.45°S; λ=106.87°E, elevation 3500 m) in Antarctica demonstrate that extremely intense electric fields (1000–5000 V/m) can be observed during snow storms. Usually the measured value of the atmospheric electric field at Vostok is about 100–250 V/m during periods with “fair weather” conditions. Actual relation between near-surface electric fields and ionospheric electric fields remain to be a controversial problem. Some people claimed that these intense electric fields produced by snowstorms or appearing before strong earthquakes can re-distribute electric potential in the ionosphere at the heights up to 300 km. We investigated interrelation between the atmospheric and ionospheric electric fields by both experimental and theoretical methods. Our conclusion is that increased near-surface atmospheric electric fields do not contribute notably to distribution of ionospheric electric potential.     

Observation of a double-onset substorm during northward interplanetary magnetic field

For the first time, a substorm event with double onsets is shown observationally under northward IMF condition in this study. Magnetic field data from ground stations and from geosynchronous satellite, and aurora data from IMAGE satellite are examined to study the substorm activity. The results show that the intensity and the spatial extent of the event are as large as those of typical substorms. Another interesting finding is that two expansion onsets seem to occur during the event. A possible mechanism for the two onsets was proposed. The energy source for the event was also discussed.     

Two-dimensional electric field measurements in the ionospheric footprint of a flux transfer event

Line-of-sight Doppler velocities from the SuperDARN CUTLASS HF radar pair have been combined to produce the first two-dimensional vector measurements of the convection pattern throughout the ionospheric footprint of a flux transfer event (a pulsed ionospheric flow, or PIF). Very stable and moderate interplanetary magnetic field conditions, along with a preceding prolonged period of northward interplanetary magnetic field, allow a detailed study of the spatial and the temporal evolution of the ionospheric response to magnetic reconnection. The flux tube footprint is tracked for half an hour across six hours of local time in the auroral zone, from magnetic local noon to dusk. The motion of the footprint of the newly reconnected flux tube is compared with the ionospheric convection velocity. Two primary intervals in the PIFs evolution have been determined. For the first half of its lifetime in the radar field of view the phase speed of the PIF is highly variable and the mean speed is nearly twice the ionospheric convection speed. For the final half of its lifetime the phase velocity becomes much less variable and slows down to the ionospheric convection velocity. The evolution of the flux tube in the magnetosphere has been studied using magnetic field, magnetopause and magnetosheath models. The data are consistent with an interval of azimuthally propagating magnetopause reconnection, in a manner consonant with a peeling of magnetic flux from the magnetopause, followed by an interval of anti-sunward convection of reconnected flux tubes.