A. V. Dmitriev

Necessary conditions for geosynchronous magnetopause crossings

[1]xa0The International Solar Terrestrial Physics database of the magnetic measurements on GOES and plasma measurements on Los Alamos National Laboratory (LANL) geosynchronous satellites is used for selection of 169 case events containing 638 geosynchronous magnetopause crossings (GMCs) in 1995 to 2001. The GMCs and magnetosheath intervals associated with them are identified using advanced methods that take into account (1) strong deviation of the magnetic field measured by GOES from the magnetospheric field, (2) high correlation between the GOES magnetic field and interplanetary magnetic field (IMF), and (3) substantial increase of the midenergy ion and electron fluxes measured by LANL. Accurate determination of the upstream solar wind conditions for the GMCs is performed using correlation of geomagnetic activity (Dst (SYM-H) index) with the upstream solar wind pressure. The location of the GMCs and associated upstream solar wind conditions are ordered in an aberrated GSM coordinate system (aGSM) with X-axis directed along the solar wind flow. In the selected data set of GMCs the solar wind total pressure Psw varies up to 100 nPa and the southward IMF Bz reaches 60 nT. We study the conditions necessary for geosynchronous magnetopause crossings using scatterplots of the GMCs in the coordinate space of Psw versus Bz. In such a representation the upstream solar wind conditions show a sharp envelope boundary beyond which no GMCs are observed. The boundary has two straight horizontal branches where Bz does not influence the magnetopause location. The first branch is located in the range of Psw = 21 nPa for large positive Bz and is associated with a regime of pressure balance. The second branch asymptotically approaches the range of Psw = 4.8 nPa under strong negative Bz, and it is associated with a regime in which the Bz influence saturates. The intermediate region of the boundary ranges from moderate negative to moderate positive IMF Bz and can be well approximated by a hyperbolic tangent function. We interpret the envelope boundary as a range of necessary upstream solar wind conditions required for the magnetopause to reach geosynchronous orbit at its closest approach to the Earth (its “perigee” location).

Magnetopause expansions for quasi‐radial interplanetary magnetic field: THEMIS and Geotail observations

We report THEMIS and Geotail observations of prolonged magnetopause (MP) expansions during long-lasting intervals of quasi-radial interplanetary magnetic field (IMF) and nearly constant solar wind dynamic pressure. The expansions were global: the magnetopause was located more than 3 RE and ~7 RE outside its nominal dayside and magnetotail locations, respectively. The expanded states persisted several hours, just as long as the quasi-radial IMF conditions, indicating steady-state situations. For an observed solar wind pressure of ~1.1-1.3 nPa, the new equilibrium subsolar MP position lay at ~14.5 RE, far beyond its expected location. The equilibrium position was affected by geomagnetic activity. The magnetopause expansions result from significant decreases in the total pressure of the high-beta magnetosheath, which we term the low-pressure magnetosheath (LPM) mode. A prominent LPM mode was observed for upstream conditions characterized by IMF cone angles less than 20 ~ 25 grad, high Mach numbers and proton plasma beta<1.3. The minimum value for the total pressure observed by THEMIS in the magnetosheath adjacent to the magnetopause was 0.16 nPa and the fraction of the solar wind pressure applied to the magnetopause was therefore 0.2, extremely small. The equilibrium location of the magnetopause was modulated by a nearly continuous wavy motion over a wide range of time and space scales.The pressure balance at the magnetopause is formed by magnetic field and plasma in the magnetosheath, on one side, and inside the magnetosphere, on the other side. In the approach of dipole earths magnetic field configuration and gas-dynamics solar wind flowing around the magnetosphere, the pressure balance predicts that the magnetopause distance R depends on solar wind dynamic pressure Pd as a power low R ~ Pd^alpha, where the exponent alpha=-1/6. In the real magnetosphere the magnetic filed is contributed by additional sources: Chapman-Ferraro current system, field-aligned currents, tail current, and storm-time ring current. Net contribution of those sources depends on particular magnetospheric region and varies with solar wind conditions and geomagnetic activity. As a result, the parameters of pressure balance, including power index alpha, depend on both the local position at the magnetopause and geomagnetic activity. In addition, the pressure balance can be affected by a non-linear transfer of the solar wind energy to the magnetosheath, especially for quasi-radial regime of the subsolar bow shock formation proper for the interplanetary magnetic field vector aligned with the solar wind plasma flow.

Dawn‐dusk asymmetry of geosynchronous magnetopause crossings

[1]xa0Geosynchronous magnetopause crossing (GMC) data were collected from literature sources from 1967 to 1993 (189 GMCs) and from the experimental data on magnetic measurements on GOES (129 GMCs) and plasma measurements on LANL (197 GMCs) geosynchronous satellites in 1994 to 2001. The dawn-dusk asymmetry of the magnetopause at geosynchronous orbit was examined by two independent methods using the collected data set of 515 GMCs. First, the large amount of accumulated GMCs permitted the revealing of a substantial dawn-dusk asymmetry in the local time (LT) distribution of the GMC occurrence probability, with a statistically significant maximum in the range from 1000 LT to 1100 LT. Second, an analysis of the dawn-dusk asymmetry dependence on the upstream solar wind conditions was performed using a scatter plot of the solar wind total pressure versus local time for various IMF Bz. There was no asymmetry revealed for large positive Bz. Under strong negative Bz we found a prominent magnetopause dawn-dusk asymmetry. The asymmetry is characterized by a shifting of the GMCs with the minimal required solar wind total pressure toward the dawn and by a significantly lower (about 3 times) solar wind pressures required for the GMCs in the dawn sector relative to the dusk sector. We found that the asymmetry cannot be attributed to the IMF orientation along the Parker spiral, which is not revealed for strongly disturbed solar wind conditions accompanying the GMCs. An application of the dawn-dusk asymmetry effect for the Chao et al. [2002] model provided a substantial increase in the model predictive capability interim of the geosynchronous magnetopause crossings. The standard deviation decreased by 20% from 0.55 RE for the initial version to 0.45 RE for the asymmetrical version of the model, with the magnetopause axis rotated by an angle of about 15° toward the dawn. The physical processes responsible for the magnetopause dawn-dusk asymmetry are discussed. We indicate the two most probable magnetospheric phenomena, which would contribute to the substantial dawn-dusk asymmetry of the magnetopause under disturbed solar wind and geomagnetic conditions. The first one is that magnetopause erosion would operate more intensively in the prenoon sector. The second phenomenon is an asymmetrical terrestrial ring current that would develop during geomagnetic storms.

Comparative study of bow shock models using Wind and Geotail observations

Wind and Geotail observed bow shock (BS) crossings were selected from the 1998 to 2001 ISTP database. We analyzed 625 case events containing 4381 Geotail BS crossings and 130 case events containing 917 Wind BS crossings. The location of the BS crossings, in the aberrated GSE coordinate system, varied over a wide range from -85 Re to 45 Re along the X-GSE axis, up to 90 Re in the perpendicular direction. ACE, Wind, and Geotail measurements were used to determine the upstream solar wind conditions in the interplanetary medium. These conditions were determined for the BS crossings in each case event by using the delay time of direct solar wind propagation from an upstream monitor to the probe satellite (Wind or Geotail). The solar wind conditions for the BS crossings varied over a wide range of dynamic pressures Pd (from 0.02 nPa to 49 nPa), IMF Bzs (from -26 nT to 23 nT), thermal/magnetic pressure ratios beta (from 0.002 to 50), and magnetosonic Mach numbers M(ms) (from 1.02 to 29). Such a wide spatial and dynamic range of BS crossings permits us to consider the different parameters that control the BS size and shape, such as the radius of curvature of the magnetopause which depends on Pd and Bz, the Alfven, sonic, and magnetosonic Mach numbers, and the IMF orientation. To study the dependence on these parameters, we compared the accuracy of the BS models formulated by Peredo et al. [1995], Russell and Petrinec [1996], Verigin et al. [2001b], and Chao et al. [2002] for the prediction of selected BS crossings observed in different bow shock regions and with various upstream solar wind conditions. It was found that the Chao et al. [2002] model had the best capability for predicting the BS crossings. The solar wind dynamic pressure and magnetosonic Mach number were determined to be the most important parameters controlling the BS size and shape. The important role of the dawn-dusk asymmetry of the bow shock tail region is emphasized. The effect of the southward IMF influence on the dayside magnetosheath thickness is revealed and discussed.

Traveling magnetopause distortion related to a large‐scale magnetosheath plasma jet: THEMIS and ground‐based observations

[1]xa0Here, we present a case study of THEMIS and ground-based observations of the perturbed dayside magnetopause and the geomagnetic field in relation to the interaction of an interplanetary directional discontinuity (DD) with the magnetosphere on 16 June 2007. The interaction resulted in a large-scale local magnetopause distortion of an “expansion – compression – expansion” (ECE) sequence that lasted for ∼15 min. The compression was caused by a very dense, cold, and fast high-βmagnetosheath plasma flow, a so-called plasma jet, whose kinetic energy was approximately three times higher than the energy of the incident solar wind. The plasma jet resulted in the effective penetration of magnetosheath plasma inside the magnetosphere. A strong distortion of the Chapman-Ferraro current in the ECE sequence generated a tripolar magnetic pulse “decrease – peak– decrease” (DPD) that was observed at low and middle latitudes by some ground-based magnetometers of the INTERMAGNET network. The characteristics of the ECE sequence and the spatial-temporal dynamics of the DPD pulse were found to be very different from any reported patterns of DD interactions with the magnetosphere. The observed features only partially resembled structures such as FTE, hot flow anomalies, and transient density events. Thus, it is difficult to explain them in the context of existing models.

A predictive model of geosynchronous magnetopause crossings

[1]xa0We have developed a model predicting whether or not the magnetopause crosses geosynchronous orbit at a given location for given solar wind pressure Psw, Bz component of the interplanetary magnetic field (IMF), and geomagnetic conditions characterized by 1 min SYM-H index. The model is based on more than 300 geosynchronous magnetopause crossings (GMCs) and about 6000 min when geosynchronous satellites of GOES and Los Alamos National Laboratory (LANL) series are located in the magnetosheath (so-called MSh intervals) in 1994–2001. Minimizing of the Psw required for GMCs and MSh intervals at various locations, Bz, and SYM-H allows describing both an effect of magnetopause dawn-dusk asymmetry and saturation of Bz influence for very large southward IMF. The asymmetry is strong for large negative Bz and almost disappears when Bz is positive. We found that the larger the amplitude of negative SYM-H, the lower the solar wind pressure required for GMCs. We attribute this effect to a depletion of the dayside magnetic field by a storm time intensification of the cross-tail current. It is also found that the magnitude of threshold for Bz saturation increases with SYM-H index such that for small negative and positive SYM-H the effect of saturation diminishes. This supports an idea that enhanced thermal pressure of the magnetospheric plasma and ring current particles during magnetic storms results in the saturation of magnetic effect of the IMF Bz at the dayside magnetopause. A noticeable advantage of the models prediction capabilities in comparison with other magnetopause models makes the model useful for space weather predictions.

Saturation of IMF Bz influence on the position of dayside magnetopause

[1]xa0The dayside magnetopause moves closer to the Earth with increasing southward IMF Bz. Is the response of magnetopause to solar wind parameters, southward IMF Bz, and dynamic pressure Dp linear or nonlinear? GOES observations on 6 April 2000 shows that the magnetopause is still outside of geosynchronous orbit even though the southward IMF Bz is greater than 25 nT and Dp is near 8 nPa. We suggest that the earthward motion of the dayside magnetopause saturates for large southward IMF Bz. Magnetosheath encounters observed by GOES satellites during 1999–2000 are used as a database for selecting a functional form of the saturation effect based on the calculations of the modified magnetopause model of Chao et al. [2002]. To obtain a relationship of the threshold of southward IMF Bz for saturation occurring as a function of Dp, an iteration procedure is used to minimize the false alarm rate (FAR) and maximize the probability of prediction (PoP). The relationship B′z = −8.1 − 12.0 × log (Dp + 1) is obtained where B′z is the threshold of IMF Bz for saturation. This relationship is applied to a modified Chao et al. [2002] model and the new model is compared against magnetosheath encounters observed by the LANL MPA instruments on 31 March 2001.

Geosynchronous magnetopause crossings on 29–31 October 2003

[1]xa0On 29–31 October 2003, numerous geosynchronous magnetopause crossings (GMCs) were identified using magnetic field data from two GOES satellites and plasma data from four Los Alamos National Laboratory (LANL) satellites. We can distinguish four long-lasting intervals, when geosynchronous satellites observed GMCs in a wide range of local time: at ∼0600–0900 UT 29 October; from ∼1000 UT 29 October to 0400 UT 30 October; from ∼1700 UT 30 October to ∼0800 UT 31 October, and at ∼1100–1300 UT 31 October. During a part of those intervals the GMCs occurred in the dawn and dusk sectors under northward interplanetary magnetic field (IMF) that indicates to magnetospheric compression by extremely high solar wind pressure. We found that at 0400–1000 UT 31 October the compression was accompanied with large-amplitude Pc5 pulsation, which can be attributed to global magnetospheric mode (cavity resonance). Multiple GMCs were revealed for the time interval of Pc5 pulsation occasion. An amplitude of the magnetopause oscillation in noon sector was estimated of about 0.26∼0.6 RE. An application of the magnetopause models enabled us studying the magnetopause dawn–dusk asymmetry, which was revealed on the main phase and in maximum of two great geomagnetic storms on 29 and 30 October. We shown that the asymmetry can be formally represented as a shifting of the dayside magnetopause toward the dusk on average distance of 0.2∼0.3 RE. Besides, in some cases the asymmetry was larger and required a shifting of about 0.4 RE. It was shown that the magnitude of the dawn–dusk asymmetry is related to the internal geomagnetic disturbances rather than to the external conditions in the IMF.

Elliptical model of cutoff boundaries for the solar energetic particles measured by POES satellites in December 2006

[1]xa0Experimental data from a constellation of five NOAA Polar Orbiting Environmental Satellites (POES), satellites were used for studying the penetration of solar energetic particles (SEP) to high latitudes during long-lasting SEP events on 5–15 December 2006. We determined cutoff latitudes for electrons with energies >100 keV and >300 keV and for protons with energies from 240 keV to >140 MeV. The large number of satellites allowed us to derive snapshots of the cutoff boundaries with 1-hour time resolution. The boundaries were fitted well by ellipses. On the basis of the elliptical approach, we developed a model of cutoff latitudes for protons and electrons in the northern and southern hemispheres. The cutoff latitude is represented as a function of rigidity, R, of particles; MLT, geomagnetic indices Dst, Kp, and AE; and dipole tilt angle PS. The model predicts tailward and duskward shifting of the cutoff boundaries in relation to intensification of the cross-tail current, field-aligned currents, and symmetrical and asymmetrical parts of the ring current. The model was applied for prediction of polar cap absorption (PCA) effects observed at high latitudes by the Canadian Advanced Digital Ionosonde network of ionosondes. It was found that the PCA effects are related mainly to intense fluxes of >2.5 MeV protons and >100 keV electrons, which contribute mostly to the ionization of ionospheric D-layer at altitudes of ∼75 to 85 km. This finding was confirmed independently by FORMOSAT-3/COSMIC observations of the SEP-associated enhancements of electron content at altitudes of ∼80 km.

Indirect estimation of the solar wind conditions in 29–31 October 2003

[1]xa0A comparative analysis of the solar wind conditions was performed for extremely disturbed event on 29–31 October 2003. It was found that the ACE and Geotail upstream monitors provided very similar data on the IMF but that plasma measurements in the SOHO CELIAS/MTOF, ACE SWEPAM, IMP 8 MIT, and Geotail CPI experiments are very different. The solar wind velocity was indirectly estimated using the time lag for propagation of such solar wind structures as interplanetary shock, Alfven waves, rotational, and tangential discontinuities from point L1 to the Earth. We found the best correspondence of the estimated velocity was with the ACE SWEPAM data, which displayed very fast (up to 2000 km/s) solar wind, while the IMP 8, Geotail, and SOHO plasma instruments are unable to measure such a fast solar wind stream. Application of the magnetopause models to a data set of numerous geosynchronous magnetopause crossings observed by GOES and LANL satellites enabled estimation of the solar wind dynamic pressure. In general the estimated pressure and density are in agreement with the solar wind plasma parameters provided by the ACE SWEPAM experiment. An estimation of the solar wind density corresponds very well to the electron density restored from the Geotail PWI data. However, during 1600–1800 UT on 29 October, 1700–1800 UT on 30 October, and 0000–0400 UT on 31 October, the estimated solar wind pressure and density are several times larger than provided by the Geotail PWI and ACE SWEPAM. A large helium abundance is considered as a possible reason for the solar wind pressure underestimation in the first case. The understated solar wind density on 30–31 October might be explained by errors in the method for restoring of the plasma data in fast solar wind (>900 km/s) accompanied with intensive fluxes (few tens of particles per cm2 s sr) of high-energy (>30 MeV) solar energetic protons.