Coordinated ISTP satellite and ground observations of morningside Pc5 waves
S. OhtaniG. RostokerKazue TakahashiV. AngelopoulosMasato NakamuraC. L. WatersH. J. SingerS. KokubunK. TsurudaWilliam HughesT. A. PotemraL. J. ZanettiJ. B. GaryA. T. Y. LuiD. J. Williams
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This paper reports the result of a coordinated data analysis of a morningside Pc 5 event observed at different altitudes in the magnetosphere and also on the ground. The event took place during 1400–1500 UT of April 29, 1993. The Geotail satellite was located in the boundary region and observed a 5‐min quasi‐periodic magnetic oscillation. The oscillation was mostly transverse to the background magnetic field. A 90° phase lag between the magnetic field and electric field variations was not clear, suggesting that the oscillation was not a standing wave and that Geotail was located in or close to the excitation region. The plasma flow vector rotated clockwise on the equatorial plane viewed from the north as expected for a magnetospheric surface wave on the morningside. At geosynchronous altitude, the GOES satellites also observed a 5‐min magnetic oscillation but with a significantly smaller amplitude than at the Geotail position. Five‐minute magnetic oscillations were also detected at Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) and Magnetometer Array for Cusp and Cleft Studies (MACCS) ground stations in the same local time sector as the satellites, even equatorward of a region 2 field‐aligned current observed by the Freja magnetometer data. From the phase analysis of ground signatures, the wave is inferred to propagate westward (antisunward) at a velocity of 18° in longitude per minute. The propagation speed mapped to the equator, 400 km/s, is in the range of the expected flow speed of the magnetosheath. It is inferred that in the present event, the Kelvin‐Helmholtz instability at the magnetopause, rather than at the inner edge of the boundary layer, excited an oscillation at the single frequency in a large area from the boundary region to deep inside the magnetosphere.Keywords:
Substorm
Oscillation (cell signaling)
Local time
Electrojet
Magnetosheath
Longitude
The entire three‐spacecraft magnetospheric plasma analyzer (MPA) data set has been examined for intervals of magnetosheath plasma at geosynchronous orbit. Over the 6 1/2 spacecraft years of data reviewed, 916 5‐min intervals of magnetosheath plasma observations were identified and cataloged. Unlike previous studies that suggested that magnetopause encounters were far more likely on the prenoon than postnoon side at geosynchronous orbit, this study examines three independent lines of evidence and finds that only a small asymmetry (in this same sense) exists. On a statistical basis, a local time of ∼1130 is both the median and mean for our magnetosheath observations; this local time is simply consistent with the effects of an aberrated solar wind direction due to the motion of the Earth around the Sun. Simultaneous multipoint observations across both sides of local noon are also consistent with a small offset due to aberration of the magnetopause. Finally, we examined the locations of the dawn‐dusk flow reversals for a number of cases where one of the geosynchronous spacecraft was outside the magnetopause in the magnetosheath flow. These flow reversals tended to occur near local noon, again indicating that in contrast to previous findings, no large asymmetry exists.
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Noon
Local time
Magnetosphere of Saturn
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The fine structure of two magnetopause crossings observed on November 10, 1977, is studied in detail as representative of a class of magnetopause observations. A so‐called magnetopause layer is observed between the magnetosheath and the magnetosphere. Changes in the magnetic field indicate that there are current sheets on each side of this magnetopause layer, and therefore does not show the smooth features of a rotational discontinuity observed on other occasions. The magnetopause layer is characterized by an irregular and a magnetosheath‐like energy distribution, part of the time with increased flow and energy compared with the magnetosheath. The flow direction deviates considerably from that of the magnetosheath. No boundary layer plasma is observed on the magnetospheric field lines inside the magnetopause layer in the cases discussed. Electromagnetic energy is dissipated at the two edges of the magnetopause layer where the current layers are observed. The two crossings are tentatively interpreted to take place on the two sides of the X line that moved because of a change of the inclination of the magnetosheath magnetic field.
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Recent studies indicated that the magnetopause indentation plays an important role in magnetosphere-ionosphere coupling. Confirmation of magnetopause indentation requires joint observations with multiple satellites. So far, there have been few magnetopause indentation events reported. In this paper, a case of magnetopause indentation induced by fast magnetosheath flow is reported with multiple spacecraft analysis based on the observations of five THEMIS probes. During the interval from 10:00 UT to 10:45 UT on 21 July 2007, when the five THEMIS probes are located near the subsolar magnetopause, a fast anti-sunward flow (with a velocity of 400 km·s-1) was observed in the magnetosheath just before THEMIS crossed the magnetopause to the magnetosphere. A magnetopause local indentation event was identified by comparing the nominal magnetopause and the tangential magnetopause plane calculated using the MVA method. In order to explore the origin of this magnetosheath fast flow, solar wind data observed by WIND satellite at L1 point were analyzed. It is found that the solar wind is very stable during this period. The Interplanetary Magnetic Field (IMF) is mainly radial and the component of the north-south direction is very small. It is speculated that the generation of this magnetosheath fast anti-sunward flow may be related to the radial IMF.
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The shape and the structure of the magnetopause are examined during several passages of the magnetopause past ATS 5 and Explorer 45 on August 4, 1972. Determination of boundary normals and comparison of observations at the two satellites indicate that the magnetopause shape in the afternoon sector was close to the shape indicated by theory. Magnetopause normals for a series of closely spaced inward/outward magnetopause passages indicate that these passages were not caused by surface waves moving along the magnetopause. Hodograms of the magnetic field observed during the first two magnetopause passages indicate that the magnetopause was a tangential discontinuity. The magnetic field changes during these passages were used to derive the magnitude and the direction of electrical currents, principally eastward, flowing in the magnetopause. The third passage was very rapid, so a detailed evaluation of the structure was not possible, but it also indicated a tangential discontinuity. A rotational discontinuity was observed in the magnetosheath followed by several magnetopause crossings where the field changed from northeastward in the magnetosheath to northward in the magnetosphere. The magnetopause currents associated with these passages flow northeastward, parallel to the magnetic field. These last observations agree with either open or closed magnetosphere models.
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Magnetic pressure inside the magnetopause is usually balanced with a sum of thermal plasma and magnetic pressures on the magnetosheath side. However, observations reveal that the magnetosheath magnetic field can be frequently larger than that in the magnetosphere (inverse magnetic field gradient across the magnetopause), and thus, the enhanced pressure from the magnetosheath side seems to be uncompensated. Such events are rare in the subsolar region, but their occurrence rate increases toward flanks. The analysis, based on statistical processing of about 35,000 THEMIS magnetopause crossings collected in the course of the years 2007–2017, shows that these events are more frequently observed under enhanced geomagnetic activity that is connected with a strong southward IMF. Case studies reveal that such a state of the magnetopause boundary layers can persist for several hours. This study discusses conditions and mechanisms keeping the pressure balance across the magnetopause under these conditions.
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Pressure gradient
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Discontinuities in the solar wind, bow shock ripples or ionized dust clouds carried by the solar wind, high speed jets (HSJs) are observed in the magnetosheath. These HSJs have typically a Vx component larger than 200 km s-1 and their dynamic pressure can be a few times the solar wind dynamic pressure. We use a conjunction of Cluster and MMS, crossing simultaneously the magnetopause, to study the characteristics of these HSJs and their impact on the magnetopause. Over one hour-fifteen minutes interval in the magnetosheath, Cluster observed 21 HSJs. During the same period, MMS observed 12 HSJs and entered the magnetosphere several times. A jet was observed simultaneously by both MMS and Cluster and it is very likely that they were two distinct HSJs. TDuring this period, two and six magnetopause crossings were observed respectively on Cluster and MMS with a significant angle between the observation and the expected normal deduced from models. The angles observed range between from 11° up to 114°. One inbound magnetopause crossing observed by Cluster (magnetopause moving out at 142 km s-1) was observed simultaneous to an outbound magnetopause crossing observed by MMS (magnetopause moving in at -83 km s-1), showing that the magnetopause can have multiple local indentation places, most likely independent from each other. Under the continuous impacts of HSJs, the magnetopause is deformed significantly and can even move in opposite directions at different places. It can therefore not be considered as a smooth surface anymore but more as surface full of local indents. Four dust impacts were observed on MMS, although not at the time when HSJs are observed, showing that dust clouds would have been present during the observations. No dust cloud in the form of Interplanetary Field Enhancements was however observed in the solar wind which may exclude large clouds of dust as a cause of HSJs. Radial IMF and Alfven Mach number above 10 would fulfil the criteria for the creation of bow shock ripples and the subsequent crossing of HSJs in the magnetosheath.
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A model of the magnetosheath structure proposed in a recent paper from the authors is extended to estimate the magnetopause stand-off distance from solar wind data. For this purpose, the relationship of the magnetopause location to the magnetosheath and solar wind parameters is studied. It is shown that magnetopause erosion may be explained in terms of the magnetosheath magnetic field penetration into the magnetosphere. The coefficient of penetration (the ratio of the magnetospheric magnetic field depression to the intensity of the magnetosheath magnetic field Bmâ¥z=âBmsin2Θ/2, is estimated and found approximately to equal 1. It is shown that having combined a magnetosheath model presented in an earlier paper and the magnetosheath field penetration model presented in this paper, it is possible to predict the magnetopause stand-off distance from solar wind parameters.Key words. Magnetospheric physics · Magnetopause · Cusp and boundary layers-Magnetosheath
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Abstract. The paper analyses one long-term pass (26 August 2007) of the THEMIS spacecraft across the dayside low-latitude magnetopause. THEMIS B, serving partly as a magnetosheath monitor, observed several changes of the magnetic field that were accompanied by dynamic changes of the magnetopause location and/or the structure of magnetopause layers observed by THEMIS C, D, and E, whereas THEMIS A scanned the inner magnetosphere. We discuss the plasma and the magnetic field data with motivation to identify sources of observed quasiperiodic plasma transients. Such events at the magnetopause are usually attributed to pressure pulses coming from the solar wind, foreshock fluctuations, flux transfer events or surface waves. The presented transient events differ in nature (the magnetopause surface deformation, the low-latitude boundary layer thickening, the crossing of the reconnection site), but we found that all of them are associated with changes of the magnetosheath magnetic field orientation and with enhancements or depressions of the plasma density. Since these features are not observed in the data of upstream monitors, the study emphasizes the role of magnetosheath fluctuations in the solar wind-magnetosphere coupling.
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Abstract. A model of the magnetosheath structure proposed in a recent paper from the authors is extended to estimate the magnetopause stand-off distance from solar wind data. For this purpose, the relationship of the magnetopause location to the magnetosheath and solar wind parameters is studied. It is shown that magnetopause erosion may be explained in terms of the magnetosheath magnetic field penetration into the magnetosphere. The coefficient of penetration (the ratio of the magnetospheric magnetic field depression to the intensity of the magnetosheath magnetic field Bm⊥z=–Bmsin2Θ/2, is estimated and found approximately to equal 1. It is shown that having combined a magnetosheath model presented in an earlier paper and the magnetosheath field penetration model presented in this paper, it is possible to predict the magnetopause stand-off distance from solar wind parameters.Key words. Magnetospheric physics · Magnetopause · Cusp and boundary layers-Magnetosheath
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Abstract Magnetopause position is controlled mainly by the solar wind dynamic pressure and north‐south interplanetary magnetic field component and these quantities are included in different empirical magnetopause models. We have collected about 50,000 of dayside magnetopause crossings observed by THEMIS in course of 2007–2019 and compared the observed magnetopause position with model prediction. The difference between observed and predicted magnetopause radial distance, R obs − R mod is used for quantifying the model‐observation agreement. Its median values are well predicted for cases up to R obs ≈ 12 R E for all models but higher positive deviations are found for larger magnetopause distances, mainly under a nearly radial field and low dynamic pressure. The analysis reveals their connection with transient magnetopause displacements caused by strong sunward flows in the magnetosheath. We discuss the possible origin of the observed magnetosheath flow switching in terms of the interaction of magnetosheath jets with the magnetopause.
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