Substorm related patterns of electron density and conductance changes in the auroral zone.

<p>Enhanced precipitation of magnetospheric energetic particles during substorms increases ionospheric electron density and conductance. Such enhancements, which have timescales of a few hours, are not reproduced by the current ionospheric models. We use linear prediction filter technique to reconstruct the substorm-related response of electron densities at different altitudes and ionospheric conductances from long-term observations made by the European Incoherent SCATer (EISCAT) radar located at Tromso. To characterise the intensity of substorm injection at a 5min time step we use the midlatitude positive bay (MPB) index which basically responds to the substorm current wedge variations. We build response functions (LPF filters) between T0-1h and T0+4hrs (T0 is a substorm onset time) in different MLT sectors to estimate the magnitude and delays of the ionospheric density response at different altitudes. The systematic and largest relative substorm related changes are mostly observed in the lowest part of E and in D regions. The duration of the response is about 3 hours. It starts and reaches maximum magnitude near midnight, from which it mainly propagates toward east, where it decays when passing into the noon-evening sector. Such MLT structure corresponds to the drift motion of the injected high energy electron cloud in the magnetosphere. Model performance is better at the midnight-morning sectors (CC~0.6-0.65), where the response is larger, and it is getting worse at the noon-evening sector (CC~0.3-0.5). We also discuss the changes of effective electron energy spectra with the substorm time and MLT and compare the behaviors of global ionization, auroral absorption and conductance patterns as it propagates azimuthally from midnight along the auroral zone following after T0 time. Research was supported by RFBR grants №19-35-90054 and №19-05-00072 and MON grant №2020-220-08-6949.</p>

SUPERSUBSTORM ON 28 MAY 2011 – GEOMAGNETIC EFFECTS IN THE GLOBAL SCALE

For this analysis, we selected the supersubstorm (SSS) occurred during the strong magnetic storm on 28 May 2011 (SYM/H~100 nT). The ground-based magnetic effects of SSS have been studied basing on the data from the global SuperMAG, INTERMAGNET and IMAGE magnetometer networks, as well as on the magnetic measurements by the ionospheric satellite AMPERE system. According to the SML- index behavior, the SSS event maximum was identified at ~09:00 UT on 28 May 2011 (SML= ~-2600 nT). The SSS occurred during the passage of the magnetic cloud in the solar wind. Before the SSS, the BZ component of the Interplanetary Magnetic Field (IMF) was negative, the IMF BY component was positive, and the local jump in the solar wind dynamic pressure was registered. We found that the SSS developed in the magnetosphere in the global scale. A strong westward electrojet was observed at auroral latitudes from the evening side to the dayside. In contrast to the typical scenario of a classical substorm, a very intense eastward electrojet was detected in the afternoon-evening sector. That may be a result of the formation of an additional partial ring current during the supersubstorm.

Zonal velocity of the equatorial plasma bubbles over Kolhapur, India

This paper presents the observations of zonal drift velocities of equatorial ionospheric plasma bubbles and their comparison with model values. These velocities are determined by nightglow OI 630.0 nm images. The nightglow observations have been carried out from the low latitude station Kolhapur (16.8° N, 74.2° E; 10.6° N dip lat.) during clear moonless nights. Herein we have presented the drift velocities of equatorial plasma bubbles for the period of February–April 2011. Out of 80 nights, 39 showed the occurrence of equatorial plasma bubbles (49%). These 39 nights correspond to magnetically quiet days (ΣKp < 26). The average eastward zonal velocities (112 ± 10 m s−1) of equatorial plasma bubbles increased from evening sector to 21:00 IST (Indian Standard Time = Universal Time + 05:30:00 h), reach maximum about 165 ± 30 m s−1 and then decreases with time. The calculated velocities are in good agreement with that of recently reported values obtained with models with occasional differences; possible mechanisms of which are discussed.

The polar cliff in the morning sector of the ionosphere

By "polar cliff" we mean the steep increase in the ionization density observed in the morning sector of the polar ionosphere. Here the properties of this remarkable feature are investigated. The data set consists of electron density and temperature measurements obtained by the Dynamics Explorer 2 satellite. Only data recorded in the Northern Hemisphere winter are considered (solar zenith angle ≥ 90°). We find that for moderately disturbed conditions, the foot of the polar cliff is located below 60° invariant latitude. Here, within about 4°, the density increases by a factor of 4, on average. The actual location of the polar cliff depends primarily on the level of geomagnetic activity, its associated density increase on geographic longitude and altitude. As to the longitudinal variations, they are attributed to asymmetries in the background ionization density at middle latitudes. Using a superposed epoch type of averaging procedure, mean latitudinal profiles of the polar cliff and the associated electron temperature changes are derived. Since these differ significantly from those derived for the afternoon/evening sector, we conclude that the subauroral ionospheric trough does not extend into the morning sector. As to the origin of the polar cliff in the morning sector, local auroral particle precipitation should play only a secondary role.

Occurrence of Equatorial Plasma Bubbles during Intense Magnetic Storms

An important issue in low-latitude ionospheric space weather is how magnetic storms affect the generation of equatorial plasma bubbles. In this study, we present the measurements of the ion density and velocity in the evening equatorial ionosphere by the Defense Meteorological Satellite Program (DMSP) satellites during 22 intense magnetic storms. The DMSP measurements show that deep ion density depletions (plasma bubbles) are generated after the interplanetary magnetic field (IMF) turns southward. The time delay between the IMF southward turning and the first DMSP detection of plasma depletions decreases with the minimum value of the IMFBz, the maximum value of the interplanetary electric field (IEF)Ey, and the magnitude of the Dst index. The results of this study provide strong evidence that penetration electric field associated with southward IMF during the main phase of magnetic storms increases the generation of equatorial plasma bubbles in the evening sector.

Statistical properties of Joule heating rate, electric field and conductances at high latitudes

Statistical properties of Joule heating rate, electric field and conductances in the high latitude ionosphere are studied by a unique one-month measurement made by the EISCAT incoherent scatter radar in Tromsø (66.6 cgmlat) from 6 March to 6 April 2006. The data are from the same season (close to vernal equinox) and from similar sunspot conditions (about 1.5 years before the sunspot minimum) providing an excellent set of data to study the MLT and Kp dependence of parameters with high temporal and spatial resolution. All the parameters show a clear MLT variation, which is different for low and high Kp conditions. Our results indicate that the response of morning sector conductances and conductance ratios to increased magnetic activity is stronger than that of the evening sector. The co-location of Pedersen conductance maximum and electric field maximum in the morning sector produces the largest Joule heating rates 03–05 MLT for Kp≥3. In the evening sector, a smaller maximum occurs at 18 MLT. Minimum Joule heating rates in the nightside are statistically observed at 23 MLT, which is the location of the electric Harang discontinuity. An important outcome of the paper are the fitted functions for the Joule heating rate as a function of electric field magnitude, separately for four MLT sectors and two activity levels (Kp<3 and Kp≥3). In addition to the squared electric field, the fit includes a linear term to study the possible anticorrelation or correlation between electric field and conductance. In the midday sector, positive correlation is found as well as in the morning sector for the high activity case. In the midnight and evening sectors, anticorrelation between electric field and conductance is obtained, i.e. high electric fields are associated with low conductances. This is expected to occur in the return current regions adjacent to auroral arcs as a result of ionosphere-magnetosphere coupling, as discussed by Aikio et al. (2004) In addition, a part of the anticorrelation may come from polarization effects inside high-conductance regions, e.g. auroral arcs. These observations confirm the speculated effect of small scale electrodynamics, which is not included in most of the global modeling efforts of Joule heating rate.

E-region decameter-scale plasma waves observed by the dual TIGER HF radars

The dual Tasman International Geospace Environment Radar (TIGER) HF radars regularly observe E-region echoes at sub-auroral magnetic latitudes 58°–60° S including during geomagnetic storms. We present a statistical analysis of E-region backscatter observed in a period of ~2 years (late 2004–2006) by the TIGER Bruny Island and Unwin HF radars, with particular emphasis on storm-time backscatter. It is found that the HF echoes normally form a 300-km-wide band at ranges 225–540 km. In the evening sector during geomagnetic storms, however, the HF echoes form a curved band joining to the F-region band at ~700 km. The curved band lies close to the locations where the geometric aspect angle is zero, implying little to no refraction during geomagnetic storms, which is an opposite result to what has been reported in the past. The echo occurrence, Doppler velocity, and spectral width of the HF echoes are examined in order to determine whether new HF echo types are observed at sub-auroral latitudes, particularly during geomagnetic storms. The datasets of both TIGER radars are found to be dominated by low-velocity echoes. A separate population of storm-time echoes is also identified within the datasets of both radars with most of these echoes showing similar characteristics to the low-velocity echo population. The storm-time backscatter observed by the Bruny Island radar, on the other hand, includes near-range echoes (r<405 km) that exhibit some characteristics of what has been previously termed the High Aspect angle Irregularity Region (HAIR) echoes. We show that these echoes appear to be a storm-time phenomenon and further investigate this population by comparing their Doppler velocity with the simultaneously measured F- and E-region irregularity velocities. It is suggested that the HAIR-like echoes are observed only by HF radars with relatively poor geometric aspect angles when electron density is low and when the electric field is particularly high.

On the use of IMAGE FUV for estimating the latitude of the open/closed magnetic field line boundary in the ionosphere

A statistical comparison of the latitude of the open/closed magnetic field line boundary (OCB) as estimated from the three far ultraviolet (FUV) detectors onboard the IMAGE spacecraft (the Wideband Imaging camera, WIC, and the Spectrographic Imagers, SI-12 and SI-13) has been carried out over all magnetic local times. A total of over 400 000 OCB estimations were compared from December 2000 and January and December of 2001–2002. The modal latitude difference between the FUV OCB proxies from the three detectors is small, <1°, except in the predawn and evening sectors, where the SI-12 OCB proxy is found to be displaced from both the SI-13 and WIC OCB proxies by up to 2° poleward in the predawn sector and by up to 2° equatorward in the evening sector. Comparing the IMAGE FUV OCB proxies with that determined from particle precipitation measurements by the Defense Meteorological Satellites Program (DMSP) also shows systematic differences. The SI-12 OCB proxy is found to be at higher latitude in the predawn sector, in better agreement with the DMSP OCB proxy. The WIC and SI-13 OCB proxies are found to be in better agreement with the DMSP OCB proxy at most other magnetic local times. These systematic offsets may be used to correct FUV OCB proxies to give a more accurate estimate of the OCB latitude.