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High-speed streams (HSSs) are believed to be only slightly affected by different interactions on their path from the Sun to Earth and thus the analysis of their observations can provide information on the structure and temporal variations of the magnetic field and plasma parameters at the source region. We have chosen three coronal holes supplying 14 HSSs recorded by Wind in 2008. For each HSS, we have calculated the average magnetic field and plasma parameters as well as power spectral densities (PSDs) of magnetic field fluctuations in the MHD and kinetic ranges to investigate their long- and short-term variations. We suggest that long-term variations are connected with a time evolution of the source region on the time scale of solar rotations. On the other hand, the short-term variations would reflect a longitudinal structure of the coronal hole. Our study reveals that coronal holes are very stable source of HSSs and their temporal evolution on short- and long-time scales is negligible. This is true for the average parameters as well as for the fluctuation power and PSDs. Observed correlations between bulk and/or thermal velocity and PSD parameters are consistent with already published results. We suggest that they do not originate in the source region but they can be mostly attributed to interaction with the ambient slow wind that affects even the HSS core.

The geomagnetic activity can modulate the number and energy fluxes of precipitation and their spatial distributions. Most previous studies examined precipitation in terms of energy spectrum types associated with quasi-static potential structures (QSPS) acceleration, Alfvénic acceleration, and wave scattering under various geomagnetic conditions. In this study, we instead categorize precipitation according to energy channels of particles. The spatial distribution of the precipitation for various energy channels is also derived under different geomagnetic conditions. Our results indicate that regardless of active and quiet times, low-energy (high-energy) precipitation is mostly distributed on the dayside (nightside). By comparing with past results, we infer that electron precipitation is mainly caused by QSPS and Alfvénic acceleration for most cases; however, the high-energy electrons during quiet times are predominantly created by wave scattering. For high-energy precipitation, the dawn-dusk asymmetry of the spatial distribution during active times is found to be opposite of that during quiet times. Based on their spatial distributions, we suggest that the high-energy precipitation during quiet times is dominated by the curvature and gradient drifts, while that during active times is mainly affected by physical processes related to substorms in the magnetotail.

We use the Time History of Events and Macroscale Interactions during Substorms data to investigate the magnetic field structure just outside the magnetopause and its time evolution for radial interplanetary magnetic field (IMF) events. When the magnetic field drapes around the magnetopause in the magnetosheath region, an asymmetric magnetic field orientation in different hemispheres is expected. Our two-case study reveals some conflicts with the predicted draped field configuration in the Southern Hemisphere. The magnetosheath Bz component had a different sign depending on the upstream IMF Bx component's polarity at the beginning of the radial IMF intervals. With time, the observed Bz became northward in both cases with increasing positive values through the events. The increasing value of the Bz component may be explained by two possible mechanisms: by a change of the upstream IMF and by a reconnection between magnetosheath and magnetospheric field lines. Our study shows that both mechanisms contributed to the observed changes. Thus, there was a correlation between the change of the upstream IMF conditions and an increase in the magnetosheath northward magnetic field component. The observed formation of the boundary layer near the magnetopause proves that the reconnection process was ongoing at least for a part of the time. We suggest two possible reconnection scenarios: one near subsolar point and another tailward of the one cusp due to lobe reconnection. The asymmetry of reconnection locations causes rearrangement of the magnetic field structure near the magnetopause and turns the observed magnetosheath Bz component even further into positive values.

This paper presents a method for taking into account changes of solar wind parameters in the foreshock using global MHD simulations. We simulate four events with very distant subsolar magnetopause crossings that occurred during quasi-radial interplanetary magnetic field (IMF) intervals lasting from one to several hours. Using previous statistical results, we suggest that the density and velocity in the foreshock cavity decrease to ∼60% and ∼94% of the ambient solar wind values when the IMF cone angle falls below 50°. This diminishes the solar wind dynamic pressure to 53% and causes a corresponding magnetospheric expansion. We change the upstream solar wind parameters in a global MHD model to take these foreshock effects into account. We demonstrate that the modified model predicts magnetopause distances during radial IMF intervals close to those observed by THEMIS. The strong total pressure decrease in the data seems to be a local, rather than a global, phenomenon. Although the simulations with decreased solar wind pressure generally reproduce the observed total pressure in the magnetosheath well, the total pressure in the magnetosphere often agrees better with results for nonmodified boundary conditions. The last result reveals a limitation of our method: we changed the boundary conditions along the whole inflow boundary, although a more correct approach would be to vary parameters only in the foreshock. A model with the suggested global modification of the boundary conditions better predicts the location of part of the magnetopause behind the foreshock but may fail in predicting the rest of the magnetopause.

We present the first observation of the Kelvin-Helmholtz (KH) rolled-up vortex at the dayside magnetopause layers under a radial interplanetary magnetic field (IMF). The study uses measurements of four THEMIS probes aligned along the YGSE axis about 10 RE upstream of the Earth and located in different regions of the near-Earth environment. THEMIS C and A serve as monitors of the quiet solar wind and fluctuating magnetosheath conditions, respectively, and THEMIS D and E observe the magnetopause and low-latitude boundary layer (LLBL) crossings. The analysis shows: (1) a radial IMF changes to the southward pointing magnetosheath magnetic field; (2) dayside reconnection forms the thin but dense LLBL; (3) a large velocity shear at the LLBL inner edge excites a train of KH waves; and (4) in spite of a short path from the subsolar point (≈5 RE), one of KH waves exhibits all features of a fully developed rolled-up vortex.

Time History of Events and Macroscale Interactions during Substorms (THEMIS) data are used to investigate the magnetic field structures in the vicinity of the magnetopause. Generally, the tendency that the farther away from the Earth, the weaker the detected magnetic field is expected inside the dayside magnetopause. Here we show two cases which conflict with the expectation that the magnetic field gradient direction reverses from inward to outward in a short time interval. After a further analysis, it is found that the THEMIS probes encountered a magnetopause indentation moving along the magnetopause towards the dawn in one case, and for the other case, they crossed an evolutive indented magnetopause that was produced locally and then recovered to its normal state. These two magnetopause indentation may be related with the fast magnetosheath flow. Accordingly, we suppose that the fast magnetic gradient direction reverse is caused by the abnormal magnetic field distribution adjacent to the deformed magnetopause.

In previous studies, the magnetic field structures in the solar wind and in the magnetosheath usually have a high dependence on only the Y component. In this study, we compared the simultaneous data from OMNI database, THEMIS-B in the solar wind, and THEMIS-C in the magnetosheath to investigate the relationship between the magnetic field structures in the solar wind and in the magnetosheath under the radial IMF conditions. We find that B z has a better correlation than B y between the magnetic fields in the two regions, and the correlation for B x is poor. When the magnetic field transits through the bow shock, the magnetic field will be enhanced in all components, especially in B y . After that, B x will divert to the Y and Z components, but the diversion to B y will be larger than that to B z in this case.

For decades, unusual locations of the magnetopause under radial interplanetary magnetic field (IMF) have been reported in many studies using in situ satellite observations. These studies have shown that the magnetopause is expanded either over all magnetic local times (MLTs) on the dayside (global expansion) or just near the noon (bullet-like expansion) during radial IMF conditions. With limited observations near the magnetopause, the type of the magnetopause expansion is still undetermined. In this study, 19 years of dayside geosynchronous magnetic field data obtained from Geostationary Operational Environmental Satellites (GOES) are statistically processed to infer the shape of the dayside magnetopause under radial IMF conditions. The MLT distributions of geosynchronous magnetic fields for radial IMF are compared with those for northward IMF with the correction of solar wind dynamic pressure distributions between two different IMF conditions. After removal of the solar wind dynamic pressure effect, we have found that the geosynchronous magnetic field for radial IMF is smaller than that for northward IMF over almost all MLTs on the dayside regardless of magnetic latitudes and seasons. The difference between geosynchronous magnetic fields for the northward and radial IMFs supports the suggestion that the dayside magnetopause is globally expanded during the radial IMF conditions.

The magnetopause location is generally believed to be determined by the solar wind dynamic pressure and by the sign and value of the interplanetary magnetic field vertical ( B Z ) component. The contribution of other parameters is usually considered to be minor or negligible near the equatorial plane. Recent papers have shown a magnetopause expansion during intervals of a nearly radial IMF but our ability to predict the magnetopause location under steady or slowly changing upstream conditions remains rather weak even if the effect of radial magnetic field is considered. We present a statistical study based on more than 10,000 magnetopause crossings identified in the THEMIS data in the course of the last half of the solar cycle. The observed magnetopause locations are compared with an empirical magnetopause model of Shue et al. (1997) and the sources of differences between observations and model predictions are analyzed. This analysis reveals that the magnetopause location depends on the solar activity being more compressed during the solar maximum. Furthermore, we have found that, beside the solar wind dynamic pressure and vertical magnetic field component, the solar wind speed and ionospheric conductivity ( F 10.7 used as a proxy) are important physical quantities controlling this compression.

Whistler mode chorus waves generally occur outside the plasmapause in the magnetosphere. The most striking feature of the waves is their occurrence in discrete elements. One of the parameters that describe the discrete elements is the repetition period (Trp), the time between consecutive elements. The Trp has not been studied statistically before. We use high-resolution waveform data to derive distributions of Trp for different local times. We find that the average Trp for the nightside (0.56 s) and dawnside (0.53 s) are smaller than those for the dayside (0.81 s) and duskside (0.97 s). Through a comparison with the background plasma and magnetic fields, we also find that the total magnetic field and temperature are the main controlling factors that affect the variability of Trp. These results are important for understanding the generation mechanism of chorus and choosing parameters in simulations that model the acceleration and loss of electrons by wave-particle interactions.

We investigate the L-MLT (i.e., L-shall versus magnetic local time) distributions of electromagnetic ion cyclotron (EMIC) waves, using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) data from 2009 to 2014, under prolonged (more than four hours) intervals of northward and southward interplanetary magnetic fields (IMFs). H-band EMIC waves show high occurrence in the dawn-to-postnoon sector (0400-1600 MLT) at L > ~7 for northward IMFs. On the other hand, for southward IMFs H-band EMIC waves occur in the morning-to-afternoon sector (0700-1500 MLT), and no strong wave activity is found in the dawn sector (0400-0700 MLT). H-band EMIC waves are frequently observed in the region where the ambient temperature anisotropy for the energetic ions (~1-25 keV) is high. He-band EMIC waves show high occurrence in the noon-to-dusk sector (1100-2000 MLT) for both IMF BZ conditions. In the dusk sector, however, the high occurrence region is shifted into the inner L-shell for southward IMFs (L < 8) in contrast to that observed for northward IMFs (L < 10). He-band EMIC waves are shown to be generated in the region where the total plasma density is high (especially near the plasmapause). The occurrence rates of EMIC waves are lower overall for southward IMFs than for northward IMFs for both H-band and He-band waves. We suggest that an increase of heavy ion (i.e., He+ and O+) concentration is likely associated with low occurrence rates in both frequency bands for southward IMFs.

Contact discontinuities (CDs) are discontinuities that have the continuous magnetic fields linked between two sides but no plasma flow across their surfaces. Due to a lack of the CDs evidence in observations, their stable existence is still under debate. CDs are not expected to be observed because of rapid plasma diffusion along the magnetic fields across their surfaces. In this study, we propose a feasible procedure with single-satellite data for selecting CD based on MHD jump conditions. We find two CD events, one in the solar wind and another in magnetosphere. The electron temperature is smaller than the ion temperature, and the magnetic pressure is larger than the thermal pressure in both CD events. The widths of the transition in the CD events are estimated to be 4.3 and 11.7 ion gyroradii. These two events might be CDs. Multiple spacecraft analyses are needed for further advancement in this topic.

The radial interplanetary magnetic field (IMF) is one of the special solar wind conditions when the orientation of the IMF is aligned with the solar wind velocity. In this study, we reexamine the solar wind condition during the long-duration radial IMF (>4 h) using the OMNI solar wind data. During the events, the IMF magnitude, solar wind speed, density, and especially its temperature are depressed in comparison with their yearly averages. In contrast to previous studies, we have found that the total time of the radial IMF per year does not change with solar activity. MHD simulation models failed to predict the location of the magnetopause under the radial IMF condition. A part of the inaccuracy is due to the use of assumed solar wind parameters in the simulations. Here we provide MHD modelers with the real solar wind parameters for simulations of the radial IMF.

The present work has investigated the configuration of field-aligned currents (FACs) during a long period of radial interplanetary magnetic field (IMF) on 19 May 2002 by using high-resolution and precise vector magnetic field measurements of the CHAMP satellite. During the interest period IMF By and Bz are weakly positive and Bx keeps pointing to the Earth for almost 10 hours. The geomagnetic indices Dst is about -40 nT and AE about 100 nT on average. The cross polar cap potential calculated from Assimilative Mapping of Ionospheric Electrodynamics and derived from DMSP observations have average values of 10-20 kV. Obvious hemispheric differences are shown in the configurations of FACs on the day and nightside. At the south pole FACs diminish in intensity to magnitudes of about 0.1 μA/m2, the plasma convection maintains two cell flow pattern, and the thermospheric density is quiet low. However, there are obvious activities in the northern cusp region. One pair of FACs with a downward leg toward the pole and upward leg on the equatorward side emerge in the northern cusp region, exhibiting opposite polarity to FACs typical for duskward IMF orientation. An obvious sunward plasma flow channel persists during the whole period. These ionospheric features might be manifestations of an efficient magnetic reconnection process occurring in the northern magnetospheric flanks at high latitude. The enhanced ionospheric current systems might deposit large amount of Joule heating into the thermosphere. The air densities in the cusp region get enhanced and subsequently propagate equatorward on the dayside. Although geomagnetic indices during the radial IMF indicate low level activity, the present study demonstrates that there are prevailing energy inputs from the magnetosphere to both the ionosphere and thermosphere in the northern polar cusp region.

[1] The magnetopause is the thin boundary between the solar wind and the Earth's magnetic field. The magnetopause moves earthward as the total pressure just inside the magnetopause is reduced or the total pressure just outside the magnetopause is enhanced. It has been suggested for more than four decades that field-aligned currents, created by magnetic reconnection for southward interplanetary magnetic field, reduce the magnetic pressure just inside the magnetopause. Under the assumption of a constant total pressure just outside the magnetopause, the magnetopause consequently moves earthward. Here we show that the reduced magnetic pressure just inside the magnetopause is insufficient to account for its earthward motion to its real location. For this to happen, an enhancement in the total pressure just outside the magnetopause is required. Our observations also reveal that the enhanced thermal pressure is the major contributor to the total pressure enhancement.

In Taiwan, research and education of solar terrestrial sciences began with a ground-based ionosonde operated by Ministry of Communications in 1952 and courses of ionospheric physics and space physics offered by National Central University (NCU) in 1959, respectively. Since 1990, to enhance both research and education, the Institute of Space Science at NCU has been setting up and operating ground-based observations of micropulsations, very high-frequency radar, low-latitude ionospheric tomography network, high-frequency Doppler sounder, digital ionosondes, and total electron content (TEC) derived from ground-based GPS receivers to study the morphology of the ionosphere for diurnal, seasonal, geophysical, and solar activity variations, as well as the ionosphere response to solar flares, solar wind, solar eclipses, magnetic storms, earthquakes, tsunami, and so on. Meanwhile, to have better understanding on physics and mechanisms, model simulations for the heliosphere, solar wind, magnetosphere, and ionosphere are also introduced and developed. After the 21 September 1999 Mw7.6 Chi–Chi earthquake, seismo-ionospheric precursors and seismo-traveling ionospheric disturbances induced by earthquakes become the most interesting and challenging research topics of the world. The development of solar terrestrial sciences grows even much faster after National Space Origination has been launching a series of FORMOSAT satellites since 1999. ROCSAT-1 (now renamed FORMOSAT-1) measures the ion composition, density, temperature, and drift velocity at the 600-km altitude in the low-latitude ionosphere; FORMOSAT-2 is to investigate lightning-induced transient luminous events, polar aurora, and upper atmospheric airglow, and FORMOSAT-3 probes ionospheric electron density profiles of the globe. In the near future, FORMOSAT-5 and FORMOSAT-7/COSMIC-2 will be employed for studying solar terrestrial sciences. These satellite missions play an important role on the recent development of solar terrestrial sciences in Taiwan.

Although the average magnetopause is ∼10 R E from the Earth, the magnetopause moves inside the geosynchronous orbit during extreme solar wind conditions. Under these circumstances some geosynchronous satellites suddenly enter the magnetosheath and are exposed to the plasma and fields of the magnetosheath. In this study we evaluate the predictive capabilities of magnetopause location models in forecasting geosynchronous magnetopause crossings. We predict periods during which geosynchronous satellites enter the magnetosheath using the Petrinec and Russell [1996] and Shue et al. [1998] magnetopause location models driven by data from Interplanetary Monitor Platform (IMP) 8. These predictions are then verified with in situ observations from Geosynchronous Operational Environment Satellite (GOES) 5, 6, and 7. We estimate the false alarm rate, probability of detection, and probability of false prediction for the two models. The estimation shows that false alarm rate for a forecast with a 20-min separation cadence is ∼62% (80%) for the Shue et al. [1998] model (the Petrinec and Russell [1996] model). The probability of detection is very high for both prediction models. These results suggest that both models work well in predicting magnetosheath periods for geosynchronous satellites. Predictions from the models provide a prerequisite condition for geosynchronous magnetopause crossings. Further examination of unsuccessful events indicates that preconditioning by the interplanetary magnetic field B z needs to be included in the forecasting procedure for a better forecast. This finding provides us with a guide to improving future magnetopause location models.

[1] The dawn–dusk locations of reconnection in the near-earth magnetotail at the time of isolated auroral breakup are studied to clarify whether breakup is always accompanied by reconnection. The near-earth reconnection is identified by tailward plasma flows faster than 200 km/s with southward magnetic field. We first identified 66 breakups in the Polar ultraviolet imager observations of the nightside polar ionosphere. We then studied tailward flows during breakups using Geotail in situ observations of the plasma sheet between 25 and 31 RE down the tail. It was found that the dawn–dusk (Y) locations of relatively fast (≥400 km/s) tailward flows were associated with breakup magnetic local time (MLT) by a regression line of YAGSM = −5.7 × (MLT + 0.6) RE with a correlation coefficient of 0.8. Most tailward flows were observed within 5 RE of the modeled Y locations, where tailward flows occurred in 88% of the 26 cases of breakups between 22 and 0 MLT. It is thus inferred that in most cases, breakup is accompanied by tailward flow near the breakup MLT with its dawn–dusk dimension ∼10 RE. There were only two events without tailward flows in the region where flows have been expected. These two events were an earthward flow event and a traveling compression region event, which are not inconsistent with the initiation of the near-earth reconnection. Auroral breakup is thus likely to always be accompanied by near-earth reconnection near breakup MLT. It is also inferred that reconnection and breakup occur simultaneously within a few minutes, assuming a time delay between reconnection onset and the arrival of tailward flows at satellite locations.