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Wave activities in the solar wind are an important topic and magnetic clouds (MCs) are a common phenomenon in interplanetary space, though waves activities associated with MCs have not been well documented. Based on a survey of 120 MCs observed by STEREO spacecraft during the years 2007–2013, this work studies electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency in and around MCs. For total 7807 ECW events, 24% of them occurred in the regions within MCs while 76% occurred in the regions around MCs. Statistics indicate that ECWs around MCs have higher frequencies, wider bandwidths, and stronger powers relative to the waves in MCs. More ECWs, on the other hand, tend to be related to a plasma with higher temperature, lower density, and larger velocity. In particular, it is found that there exist positive power law correlations between plasma betas and the wave frequencies, bandwidths, and powers. The results imply that the plasma beta should play an important role in determining the properties of ECWs, which is consistent with previous theory studies and the recent simulation results.

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.

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 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).

[1] Geosynchronous 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.

[1] The 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.

A database, the largest one to date, of magnetosheath encounters by geosynchronous satellites during 1986-1992 and 1999-2000 and upstream observations by Wind, IMP 8, or Geotail in the solar wind are used to estimate the forecasting capability of the models of Chao et al. [2001], Shue et al. [1998], and Petrinec and Russell [1996]. For each of the 1-min resolution data points obtained by GOES spacecraft, we check the following two things: if the magnetosheath was observed by the spacecraft, and if each of the three models predicted a magnetosheath encounter by the spacecraft. Three parameters are defined to quantify the models' forecasting capability: probability of prediction (PoP), probability of detection (PoD), and false alarm rate (FAR). A higher PoP and PoD with a lower FAR imply a better forecasting model. In the 1986-1992 period we found that most of the magnetosheath encounters observed at 6.6 R-E are detected. In particular, the Chao et al. [2001] model predicts the lowest FAR compared with those of the other two models. We have also studied the magnetosheath encounters made by GOES spacecraft for the period from 1999 to 2000 using Wind and Geotail as the solar wind monitor. This independent database of magnetosheath encounters during 1999-2000 confirms our previous anticipations. The PoD of Petrinec and Russell [1996] model (94%) is much higher than those of Shue et al. [1998] (74%) and Chao et al. [2001] (84%) models. The Chao et al. [2001] model has a higher PoP than the other two models. The values of FAR for Chao et al. [2001], Shue et al. [1998], and Petrinec and Russell [1996] models are 25, 27, and 40%, respectively.