Interplanetary Field Research Articles

Why does the total pressure on the subsolar magnetopause differ from the solar wind dynamic pressure?

Based on analysis of MHD equations and the results of numerical simulation in the magneto-sheath it is demonstrated that the total pressure on the magnetopause differs from the solar wind dynamic pressure in the majority of cases. From the equation of motion it follows that the total pressure is reduced due to deflection from the Sun-Earth line. At the same time, it increases because of formation of a magnetic barrier. This result is consistent with experimentally observed expansion of the magnetosphere for the radial direction of the interplanetary magnetic field, when no magnetic barrier is formed. In this paper we compare the behavior of pressure along the Sun-Earth line for the northward and radial interplanetary field, using the results of numerical MHD simulation and observational data from THEMIS. In the isotropic MHD approximation, the difference between the total pressure on the subsolar magnetopause at northern and radial IMFs does not exceed 10–12 percent. However, in the anisotropic approximation this difference increases up to 15–20 percent. The results of anisotropic modeling well agree with observed averaged profiles of pressure components in the subsolar magnetosheath.

Interplanetary origins of moderate (−100 nT < Dst ≤ −50 nT) geomagnetic storms during solar cycle 23 (1996–2008)

The interplanetary causes of 213 moderate‐intensity (−100 nT < peak Dst ≤ −50 nT) geomagnetic storms that occurred in solar cycle 23 (1996–2008) are identified. Interplanetary drivers such as corotating interaction regions (CIRs), pure high‐speed streams (HSSs), interplanetary coronal mass ejections (ICMEs) of two types [those with magnetic clouds (MCs) and those without (nonmagnetic cloud or ICME_nc)], sheaths (compressed and/or draped sheath fields), as well as their combined occurrence were identified as causes of the storms. The annual rate of occurrence of moderate storms had two peaks, one near solar maximum and the other in the descending phase, around 3 years later. The highest rate of moderate storm occurrence was found in the declining phase (25 storms year−1). The lowest occurrence rate was 5.7 storms year−1 and occurred at solar minimum. All moderate‐intensity storms were associated with southward interplanetary magnetic fields, indicating that magnetic reconnection was the main mechanism for solar wind energy transfer to the magnetosphere. Most of these storms were associated with CIRs and pure HSSs (47.9%), followed by MCs and noncloud ICMEs (20.6%), pure sheath fields (10.8%), and sheath and ICME combined occurrence (9.9%). In terms of solar cycle dependence, CIRs and HSSs are the dominant drivers in the declining phase and at solar minimum. CIRs and HSSs combined have about the same level of importance as ICMEs plus their sheaths in the rising and maximum solar cycle phases. Thus, CIRs and HSSs are the main driver of moderate storms throughout a solar cycle but with variable contributions from ICMEs, their shocks (sheaths), and combined occurrence within the solar cycle. This result is significantly different than that for intense (Dst ≤ −100 nT) and superintense (Dst ≤ −250 nT) magnetic storms shown in previous studies. For superintense geomagnetic storms, 100% of the events were due to ICME events, while for intense storms, ICMEs, sheaths, and their combination caused almost 80% of the storms. CIRs caused only 13% of the intense storms. The typical interplanetary electric field (Ey) criteria for moderate magnetic storms were identified. It was found that ~80.1% of the storms follow the criterion of Ey ≥ 2 mV m−1 for intervals longer than 2 h. It is concluded that southward directed interplanetary magnetic fields within CIRs/HSSs may be the main energy source for long‐term averaged geomagnetic activity on Earth.

OBSERVATIONS OF SOLAR ENERGETIC PARTICLES FROM3He-RICH EVENTS OVER A WIDE RANGE OF HELIOGRAPHIC LONGITUDE

A prevailing model for the origin of ^3He-rich solar energetic particle (SEP) events attributes particle acceleration to processes associated with the reconnection between closed magnetic field lines in an active region and neighboring open field lines. The open field from the small reconnection volume then provides a path along which accelerated particles escape into a relatively narrow range of angles in the heliosphere. The narrow width (standard deviation 60°. We present the observations of the ^3He-rich event of 2010 February 7, which was detected at all three spacecraft when they spanned 136° in heliographic longitude. Measured fluences of ^3He in this event were found to have a strong dependence on longitude which is well fit by a Gaussian with standard deviation ~48° centered at the longitude that is connected to the source region by a nominal Parker spiral magnetic field. We discuss several mechanisms for distributing flare-accelerated particles over a wide range of heliographic longitudes including interplanetary diffusion perpendicular to the magnetic field, spreading of a compact cluster of open field lines between the active region and the source surface where the field becomes radial and opens out into the heliosphere, and distortion of the interplanetary field by a preceding coronal mass ejection. Statistical studies of additional ^3He-rich events detected at multiple spacecraft will be needed to establish the relative importance of the various mechanisms.

Cosmic ray geomagnetic cutoff rigidities in the magnetic field of two empirical models during a strong disturbance in November 2003: A comparison of models

Cosmic ray (CR) fluxes, which penetrate into the Earth’s magnetosphere and atmosphere from the interplanetary space, are an important factor of space weather. The geomagnetic field allows or forbids CR particles to enter into a given point in the magnetosphere depending on their energy. The geomagnetic cutoff rigidity regulates the distribution of CR fluxes in the magnetosphere. The geomagnetic cutoff rigidity (geomagnetic threshold) determination accuracy is closely related to the accuracy of the magnetospheric model used in calculations. Using a method for tracing of charged CR particle trajectories in the magnetic field of a model magnetosphere, we obtained geomagnetic thresholds for two magnetosphere empirical models (Ts01 and Ts04), constructed based on the same initial database. The Ts01 model describes the middle magnetosphere for certain conditions in the solar wind and interplanetary field. The Ts04 model pays the main attention to describing the large-scale evolution of magnetospheric currents during a storm. The theoretically obtained geomagnetic thresholds have been compared with experimental thresholds, calculated using the spectrographic global survey method based on data from the global network of CR stations. The study has been performed for the period of a strong geomagnetic storm that occurred in November 2003.

Magnetic reconnection and Kelvin–Helmholtz instabilities at the Earth's magnetopause

Kelvin–Helmholtz instability (KHI), driven by the velocity inhomogeneity at Earth's magnetopause, has been shown to play a major role in mixing the magnetospheric and the solar wind plasma during northward periods. In fact, when the magnetospheric and interplanetary magnetic fields are mostly perpendicular to the equatorial plane, KHI can develop at a low latitude without being significantly inhibited by the magnetic tension. In contrast, at a high latitude, the more complex magnetic configuration is believed to totally stabilize the instability. This intrinsic 3D dynamics is investigated in a simplified geometry showing that KHI is able to kink the magnetic field lines at a mid-latitude and to create current layers where magnetic reconnection spontaneously develops. It is shown that a mid-latitude reconnection is able to change the global topology of the magnetic field and to connect interplanetary field lines to the Earth's cups, allowing the solar wind to directly enter the magnetosphere.

Direct penetration of the interplanetary electric field to low geomagnetic latitudes and its effect on magnetotelluric sounding

Long‐term monitoring of geoelectromagnetic fields, recorded with a network of nine stations covering an area of approximately 25,000 km2at low geomagnetic latitudes in northern Chile, reveals systematic and seasonally modulated variations of geomagnetic field properties. The observed seasonal variation affects almost exclusively the east‐west magnetic field component for periods between 100 and 3000 s. The ground‐based measurements of magnetic and electric fields show statistically significant coherences with the interplanetary electric field derived from solar wind and interplanetary magnetic field data of the Advanced Composition Explorer satellite. The interplanetary electric field (IEF) penetrates the polar ionosphere from where it propagates toward equatorial latitudes by waveguide transmission, with ionosphere and solid Earth acting as conducting boundaries. Signal coherence between IEF and ground data peaks at periods of approximately 90 min and up to the four harmonics. Coherence values reach 0.4 at these periods and depend on the electromagnetic field component. They vary with season and local time. Transfer functions computed between IEF and ground‐based electric and magnetic fields show local maxima at similar periods (90 min and harmonics). The coupling between the east‐west magnetic field component and the IEF shows significant seasonal variability, much larger than the other electromagnetic field components. We conclude that the IEF drives primarily a global circuit of Pedersen currents in the ionosphere. Resulting time‐varying magnetic fields induce electric currents in the ground. Related ground‐based magnetic (primarily north‐south) and electric (primarily east‐west) signals vary coherently at all local times and seasons. Conversely, magnetic signals caused by the IEF‐driven Hall currents depend much on local time and season. We show for the first time that these ionospheric Hall currents cause no induction in the ground, but they generate magnetic signatures that are confined to the waveguide between ionosphere and Earth's surface.

How unprecedented a solar minimum was it?

The end of the last solar cycle was at least 3 years late, and to date, the new solar cycle has seen mainly weaker activity since the onset of the rising phase toward the new solar maximum. The newspapers now even report when auroras are seen in Norway. This paper is an update of our review paper written during the deepest part of the last solar minimum [1]. We update the records of solar activity and its consequent effects on the interplanetary fields and solar wind density. The arrival of solar minimum allows us to use two techniques that predict sunspot maximum from readings obtained at solar minimum. It is clear that the Sun is still behaving strangely compared to the last few solar minima even though we are well beyond the minimum phase of the cycle 23–24 transition.

Deep Solar Activity Minimum 2007 – 2009: Solar Wind Properties and Major Effects on the Terrestrial Magnetosphere

We discuss the temporal variations and frequency distributions of solar wind and interplanetary magnetic field parameters during the solar minimum of 2007 – 2009 from measurements returned by the IMPACT and PLASTIC instruments on STEREO-A. We find that the density and total field strength were significantly weaker than in the previous minimum. The Alfvén Mach number was higher than typical. This reflects the weakness of magnetohydrodynamic (MHD) forces, and has a direct effect on the solar wind–magnetosphere interactions. We then discuss two major aspects that this weak solar activity had on the magnetosphere, using data from Wind and ground-based observations: i) the dayside contribution to the cross-polar cap potential (CPCP), and ii) the shapes of the magnetopause and bow shock. For i) we find a low interplanetary electric field of 1.3±0.9 mV m−1 and a CPCP of 37.3±20.2 kV. The auroral activity is closely correlated to the prevalent stream–stream interactions. We suggest that the Alfvén wave trains in the fast streams and Kelvin–Helmholtz instability were the predominant agents mediating the transfer of solar wind momentum and energy to the magnetosphere during this three-year period. For ii) we determine 328 magnetopause and 271 bow shock crossings made by Geotail, Cluster 1, and the THEMIS B and C spacecraft during a three-month interval when the daily averages of the magnetic and kinetic energy densities attained their lowest value during the three years under survey. We use the same numerical approach as in Fairfield’s (J. Geophys. Res. 76, 7600, 1971) empirical model and compare our findings with three magnetopause models. The stand-off distance of the subsolar magnetopause and bow shock were 11.8 R E and 14.35 R E, respectively. When comparing with Fairfield’s (1971) classic result, we find that the subsolar magnetosheath is thinner by ∼1 R E. This is mainly due to the low dynamic pressure which results in a sunward shift of the magnetopause. The magnetopause is more flared than in Fairfield’s model. By contrast the bow shock is less flared, and the latter is the result of weaker MHD forces.

Solar wind density controlling penetration electric field at the equatorial ionosphere during a saturation of cross polar cap potential

The most important source of electrodynamic disturbances in the equatorial ionosphere during the main phase of a storm is the prompt penetration electric field (PPEF) originating from the high‐latitude region. It has been known that such an electric field is correlated with the magnetospheric convection or interplanetary electric field. Here we show a unique case, in which the electric field disturbance in the equatorial ionosphere cannot be interpreted by this concept. During the superstorm on Nov. 20–21, 2003, the cross polar cap potential (CPCP) saturated at least for 8.2 h. The CPCP reconstructed by Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure suggested that the PPEF at the equatorial ionosphere still correlated with the saturated CPCP, but the CPCP was controlled by the solar wind density instead of the interplanetary electric field. However, the predicted CPCPs by Hill‐Siscoe‐Ober (HSO) model and Boyle‐Ridley (BR) model were not fully consistent with the AMIE result and PPEF. The PPEF also decoupled from the convection electric field in the magnetotail. Due to the decoupling, the electric field in the ring current was not able to comply with the variations of PPEF, and this resulted in a long‐duration electric field penetration without shielding.

A real‐time forecast service for the ionospheric equatorial zonal electric field

The zonal electric field is the primary driver of two important features of the equatorial ionosphere: (1)The Equatorial Ionization Anomaly (EIA), and (2) plasma density irregularities, also known as spread‐F. During propagation through the ionosphere, communication and navigation radio signals are attenuated, delayed and scattered by these ionospheric features. Prediction of the zonal electric field is therefore a key to the real‐time specification of the ionosphere. We divide the zonal electric field into a climatological contribution plus the prompt‐penetration contribution predicted by a transfer‐function model applied to the interplanetary electric field measured by the Advanced Composition Explorer (ACE) satellite. The zonal electric field is predicted about one hour in advance, covering all local times and longitudes. The real‐time prediction is available as a Google application at http://www.geomag.us/models/PPEFM/RealtimeEF.html. The benefit of this application to space weather forecasting is twofold: As the driver of the equatorial plasma fountain, the predicted zonal electric field is a leading indicator by 2–3 h of the EIA and the Total Electron Content (TEC) of the equatorial ionosphere. Second, rapid uplift of the ionosphere by strong eastward electric field is known to induce spread‐F. Prediction of enhanced prompt penetration electric field in the eastward direction therefore enables the forecast of radio communication and navigation outages in the equatorial region.

Influence of geomagnetic disturbances on atmospheric electric field (Ez) variations at high and middle latitudes

The variations of the vertical atmospheric electric field (Ez) represent the state of the global atmospheric electric circuit, which is controlled by the world thunderstorm activity and by magnetosphere–ionosphere disturbances as well. Here we present a synthesis of our main results of the effects of the geomagnetic disturbances on the Ez variations, measured at the Earth′s surface at high and middle latitudes, which were previously published by Kleimenova et al. (2008, 2010). We studied the high latitude geomagnetic substorm effects on the Ez variations on the base of the continue Ez registrations at the polar station Hornsund (Spitsbergen). This station can map into the polar cap, auroral oval or near the border between these structures in dependence on the local time and the level of the geomagnetic activity. The high-latitude Ez variations associated with the substorm activity have been established. It was found that the Ez deviations were positive (Ez values increase) in the local morning and negative ones (Ez values decrease) in the local evening. We speculate that the direction of the Ez excursion depends on the station location relative to the positive or negative vortex of the polar ionospheric plasma convection.The Ez variations at the mid-latitude station Świder (near Warsaw) have been studied during 14 magnetic storms. To avoid the meteorological influences on the Ez measurements we used only the Ez data, obtained under the “fair weather” conditions. For the first time the main phase effect of all mentioned above magnetic storms was established in the mid-latitude atmospheric electricity variations. The strong daytime Ez negative excursions (Ez value decreases) were found in association with the simultaneous night-side magnetospheric substorm developing during the studied magnetic storms. The considered Ez deviations could be results an interplanetary electric field penetration into the magnetosphere. Another plausible reason could be related to the common ionosphere conductivity increasing due to substorm energetic electron precipitation, modifying the high-latitude ionospheric part of the global atmospheric electric circuit.

In-flight calibration of the spin axis offset of a fluxgate magnetometer with an electron drift instrument

We show that the spin axis offset of a fluxgate magnetometer can be calibrated with an electron drift instrument (EDI) and that the required input time interval is relatively short. For missions such as Cluster or the upcoming Magnetospheric Multiscale (MMS) mission the spin axis offset of a fluxgate magnetometer could be determined on an orbital basis. An improvement of existing methods for finding spin axis offsets via comparison of accurate measurements of the field magnitude is presented, that additionally matches the gains of the two instruments that are being compared. The technique has been applied to EDI data from the Cluster Active Archive and fluxgate magnetometer data processed with calibration files also from the Cluster Active Archive. The method could prove to be valuable for the MMS mission because the four MMS spacecraft will only be inside the interplanetary field (where spin axis offsets can be calculated from Alfvénic fluctuations) for short periods of time and during unusual solar wind conditions.

DETERMINING THE NORTH-SOUTH DISPLACEMENT OF THE HELIOSPHERIC CURRENT SHEET FROM CORONAL STREAMER OBSERVATIONS

Inferences based on interplanetary field measurements have suggested a statistical tendency for the heliospheric current sheet (HCS) to be displaced southward of the heliographic equator during the past four solar cycles. Here, we use synoptic maps of white-light streamer structures to determine more directly the longitudinally averaged latitude of the HCS, after separating out the contribution of streamers without magnetic polarity reversals (pseudostreamers). We find a strong tendency for the HCS to be shifted southward by a few degrees during 2007-2011, but no significant shift during the 1996-1997 sunspot minimum. Fluctuations in the magnitude and direction of the north-south shifts often occur on timescales as short as one or two Carrington rotations, as a result of changes in the streamer structures due to active region emergence. The largest shifts occurred during 2010-2011 and were on the order of –6°. Such southward displacements are consistent with the overwhelming dominance of northern-hemisphere sunspot activity during the rising phase of the current solar cycle 24, resulting in a strong axisymmetric quadrupole component whose sign at the equator matched that of the north polar field; the symmetry-breaking effect of the quadrupole was further enhanced by the weakness of the polar fields.

An MHD simulation model of time‐dependent co‐rotating solar wind

We present a treatment of observation‐based time‐dependent boundary conditions for the inner boundary sphere in the time‐dependent three‐dimensional MHD simulations of the global solar wind. With this boundary treatment, we obtain super‐Alfvenic MHD solutions of time‐dependent co‐rotating solar wind structures. The boundary variables on the inner boundary sphere, at 50 solar radii in this study, are assumed to change linearly from one instant to the next. A new feature is that, in order to maintain the divergence‐free condition of the magnetic field, the changes of the time‐dependent boundary magnetic field are expressed as the potential field in a thin shell volume. The solar magnetic field data from the Wilcox Solar Observatory (WSO) and the solar wind speed data from the interplanetary scintillation (IPS) observations at Nagoya University, Japan, are used as the input boundary data. The solar wind simulated with the time‐dependent boundary condition is compared with the near‐Earth and Ulysses in situ measurement data and the solar wind simulated with the fixed boundary condition over a 7‐month period in 1991. Reasonable agreements with the in situ measurements are obtained. The differences between the two simulations in the interplanetary field line paths are significant. The three‐dimensional time‐dependent MHD solution of the global solar wind will help enhance space weather models and other fields in heliophysics.

Effects of a magnetic cloud simultaneously observed on the equatorial ionosphere in midday and midnight sectors

An impact of a magnetic cloud on the Earth’s magnetosphere occurred at 1636 UT on 25 June, 1998, associated with a sudden increase of the solar wind density and velocity, as well as a sudden increase of the zenithal component of the interplanetary magnetic field (IMF-Bz). Following the impact of the magnetic cloud, IMF-Bz was northward (10 nT) and remained steadily strong (about 15 nT) for the next six hours. IMF-Bz turned southward at 2330 UT on 25 June, 1998, and remained strongly southward (−15 nT) for the next four hours. During the positive phase of IMF-Bz, both the Auroral index and ring current index SYM/H remained steadily low indicating complete isolation of the Earth’s magnetosphere from the solar wind and no significant changes were observed in the equatorial ionosphere. After the southward turning, the steady southward IMF-Bz permitted solar wind energy to penetrate the magnetosphere and caused the generation of a magnetic storm associated with strong auroral electrojet activity (AE index). Strong southward IMF-Bz corresponds to the dawn-dusk interplanetary electric field (eastward on the dayside and westward on the night side). The ionograms at Jicamarca (night side) showed strong spread-F and at Thumba (dayside) showed an absence of equatorial type of sporadic-E, indicating a dusk-to-dawn electric field. Thus, the observations point to an electric field opposite in direction to that expected by the prompt penetration of the interplanetary electric field. An abnormally-large Auroral index (AE) associated with the start of the storm suggests that the cause of the equatorial electric field changes is due to the disturbance dynamo effect.

Variability of the solar wind suprathermal electron strahl

At suprathermal energies, interplanetary electrons commonly exhibit a magnetic field‐aligned beam referred to as the strahl. The finite strahl width is a consequence of competition between magnetic focusing as the interplanetary field weakens with distance from the Sun, and particle scattering acting to broaden the strahl along its propagation path. We present a statistical survey of the strahl using ACE Solar Wind Electron, Proton, and Alpha Monitor (SWEPAM) measurements from 1998 to 2002. A systematic fitting algorithm was applied to pitch angle distributions to identify unidirectional or counterstreaming strahl features and to quantify beam widths and intensities. The analysis indicated that a strahl is present ≥75% of the time, while counterstreaming strahls were observed about 10% of the time. The strahl width ranges from 5° to 90°; importantly, the strahl cannot be characterized by any typical width. Within counterstreaming intervals the strahl peak intensity anticorrelates with beam width while the integrated fluxes of the two simultaneously observed strahls are similar, within a factor of 2 for 75% of counterstreaming periods. Observations are consistent with a model in which integrated strahl flux leaving the corona varies over a limited range, but the degree of beam scattering along the propagation path to 1 AU varies widely. The difference between two concurrent counterstreaming strahls is likely due to different scattering profiles along the different legs of a closed field line loop, both rooted in similar coronal regions. Narrow strahls (<20°) are strongly associated with counterstreaming intervals, as well as high‐speed streams. Strahl width can either increase or decrease with energy.

Physical implications of discrepancy between summer and winter PC indices observed in the course of magnetospheric substorms

The PC index based on a statistically justified relationship between the polar cap magnetic activity and the interplanetary electric field EKL has been derived as a value standardized for the EKL intensity regardless of season, UT and hemisphere. As a result, the summer and winter PC indices are consistent with one another under ordinary conditions. Discrepancies between the summer and winter PC indices arising in the course of magnetospheric substorms are analyzed in this paper. It is argued that the channel of enhanced conductivity, formed in the auroral oval owing to intense auroral particle precipitation, strongly improves the conditions for closure of the Region 1 field-aligned currents in the winter dark polar region but only trivially affects the conditions of the Region 1 FAC closure in the summer sunlit ionosphere. Since the coefficients describing the relationship between EKL and the polar cap magnetic activity were derived for statistically justified (i.e., mean) conditions, their application to such abnormal situation, as intense field-aligned currents in the winter dark polar region, leads to overestimation of the winter PC index. The summer and winter PC indices level off as soon as the intense auroral particle precipitation terminates and the auroral ionosphere in the winter and summer polar caps returns to the ordinary (statistically justified) state.

Extreme changes in the equatorial electrojet under the influence of interplanetary electric field and the associated modification in the low‐latitude F region plasma distribution

A case of the drastic effects of an eastward prompt penetration and a westward overshielding electric field successively affecting the daytime equatorial ionosphere during the space weather event that occurred on 24 November 2001 is presented. Under the influence of the strong eastward prompt penetration electric field starting from 11:25 Indian standard time (IST), the equatorial electrojet (EEJ) strength reached the maximum value of 225 nT at 12:42 IST, almost 7 times greater than the monthly quiet time mean at the same time. This peak EEJ value exceeds the maximum observed values during the month of November for the entire solar cycle by more than 100 nT, irrespective of quiet or disturbed conditions. Further, owing to an ensuing overshielding event that occurred during the main phase of the storm rather than the end of the main phase, this unusually large EEJ showed an equally strong polarity reversal along with a weakening of the sporadic E layer over the equator. The EEJ strength was reduced from +225 to −120 nT at ∼13:45 IST, resulting in a strong counter electrojet condition. The latitudinal variation of the F region electron density data from the CHAMP satellite reveal an ill‐developed equatorial ionization anomaly at 17:00 IST (11:24 UT) over the Indian sector due to this significant weakening of the zonal electric field. These observations showcase the significant degree to which the low‐latitude ionosphere can be affected by the interplanetary electric field.

Underestimates of magnetic flux in coupled MHD model solar wind solutions

When validated with spacecraft observations, one enduring characteristic of global MHD solar wind models is the tendency to underestimate the interplanetary magnetic flux. This study quantifies the “missing flux” problem for models used in the coordinated Center for Integrated Space Weather Modeling study of corotating interaction regions, and identifies the model parameters most strongly related to the effect. We show that two important contributions are (1) insufficient thermal pressure in the coronal model to extract the required magnetic flux and (2) numerical diffusion in the model current sheets. Using Ulysses observations, we derive a calibration for the effective temperature in the polytropic coronal Magnetohydrodynamics Around a Sphere model that produces the expected interplanetary field at high latitudes. After recalibrating, we find that a 40% discrepancy still remains in the ecliptic plane. Moreover, the problem is 5% more severe for models of the solar cycle 23 minimum than it is for models of the cycle 22 minimum. We argue that the resolution of the heliospheric current sheet strongly affects both the general underestimate and the discrepancy between the two cycles. We also argue that improved resolution of current sheets in the low corona will further reduce the effect.

Early morning enhancement in ionospheric electron density during intense magnetic storms

This study reports on ionospheric disturbances that occurred in the early morning hours in the South American–Atlantic sector during a few intense/super storm events. The events were observed at latitudes close to the southern crest of the equatorial ionization anomaly (EIA) as an unusual intensification of the F region electron density peak at local times when the EIA is not usually developed. All the events were observed at pre dawn-morning hours, under conditions of northward interplanetary geomagnetic field. Large scale traveling ionospheric disturbances that are launched during highly disturbed conditions and/or equatorward surges in the thermospheric meridional winds seem to be the most probable causes of the observed disturbances.