Topic
Subsolar point
About: Subsolar point is a research topic. Over the lifetime, 234 publications have been published within this topic receiving 7180 citations.
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Papers
TL;DR: The early ISEE orbits provided the opportunity to study the magnetopause and its environs only a few Earth radii above the subsolar point with unprecedented temporal resolution as discussed by the authors.
Abstract: The early ISEE orbits provided the opportunity to study the magnetopause and its environs only a few Earth radii above the subsolar point Measurements of complete two-dimensional ion and electron distributions every 3 or 12 s, and of three-dimensional distributions every 12 or 48 s by the LASL/MPI instrumentation on both spacecraft allow a detailed study of the plasma properties with unprecedented temporal resolution This paper presents observations obtained during four successive inbound orbits in November 1977, containing a total of 9 magnetopause crossings, which occurred under widely differing orientations of the external magnetic field The main findings are: (1) The magnetosheath flow near the magnetopause is characterized by large fluctuations, which often appear to be temporal in nature (2) Between ∼01 and ∼03R E outside the magnetopause, the plasma density and pressure often start to gradually decrease as the magnetopause is approached, in conjunction with an increase in magnetic field strength These observations are in accordance with the formation of a depletion layer due to the compression of magnetic flux tubes (3) In cases where the magnetopause can be well resolved, it exhibits fluctuations in density, and especially pressure and bulk velocity around average magnetosheath values The pressure fluctuations are anticorrelated with simultaneous magnetic field pressure changes (4) In one case the magnetopause is characterized by substantially displaced electron and proton boundaries and a proton flow direction change from upwards along the magnetopause to a direction tranverse to the geomagnetic field These features are in agreement with a model of the magnetopause described by Parker (5) The character of the magnetopause sometimes varies strongly between ISEE-1 and -2 crossings which occur ∼ 1 min apart At times this is clearly the result of highly non-uniform motions There are also cases where there is very good agreement between the structures observed by the two satellites (6) In three of the nine crossings no boundary layer was present adjacent to the magnetopause More remarkably, two of the three occurred while the external magnetic field had a substantial southward component, in clear contradiction to expectations from current reconnection models (7) The only thick (low-latitude) boundary layer (LLBL) observed was characterized by sharp changes at its inner and outer edges This profile is difficult to reconcile with local plasma entry by either direct influx or diffusion (8) During the crossings which showed no boundary layer adjacent to the magnetopause, magnetosheath-like plasma was encountered sometime later Possible explanations include the sudden formation of a boundary layer at this location right at the time of the encounter, and a crossing of an ‘inclusion’ of magnetosheath plasma within the magnetosphere (9) The flow in the LLBI is highly variable observed directions include flow towards and away from the subsolar point, along the geomagnetic field and across it, tangential and normal to the magnetopause Some of these features clearly are nonstationary The scale size over which the flow directions change exceeds the separation distance (several hundred km) of the two spacecraft
177 citations
TL;DR: The Explorer 12 data collected from the SUI detectors during its entire 112-day lifetime have been comprehensively analyzed as mentioned in this paper, and some salient features observed include confirmation of the enhancement of soft electron flux and diminution of hard electron fluxes in the outer zone during magnetically disturbed times, and general support for the result of Gringauz, Kurt, Moroz, and Shklovskii.
Abstract: The elliptic orbit satellite Explorer 12 made possible the routine survey of the heart of the outer radiation zone as well as the region of transition between the magnetosphere and interplanetary space on the sunward side of the earth by means of a variety of particle and magnetic field detectors. The data received from the SUI detectors during the entire 112-day lifetime have been comprehensively analyzed. Salient features observed include: confirmation of the enhancement of soft electron fluxes and diminution of hard electron fluxes in the outer zone during magnetically disturbed times; the outward motion of the outer zone hard electron maximum during periods of enhancement; confirmation of the general Chapman and Ferraro picture of a well-defined magnetosphere with compression of the geomagnetic field during the initial phases of magnetic storms; the existence of a layer of quasi-thermalized plasma immediately outside the magnetosphere; confirmation of the result of Frank and Van Allen that the contours of constant counting rate for electrons with E ≥ 40 kev tend to protrude outward near 90° from the subsolar point and draw closer to the earth on the night side; and general support for the result of Gringauz, Kurt, Moroz, and Shklovskii that there exists a region beyond some 8Re on the night side of the earth dominated by very-low-energy electrons.
154 citations
TL;DR: In this paper, a nonthermal component in H with a scale height of about 70 km has been observed near the limb off the subsolar point, providing a total apparent number density of 90 per cu cm.
Abstract: Reduction of data from the three Mariner 10 encounters to this date have allowed identification of helium and hydrogen as atmospheric constituents. Subsolar point densities are estimated at 4500 per cu cm for He and 8 per cu cm for the thermal component of H. A nonthermal component in H with a scale height of about 70 km has been observed near the limb off the subsolar point, providing a total apparent number density of 90 per cu cm. Upper limits on other atmospheric constituents have been reduced an order of magnitude. A very tentative identification of O has been obtained but the uncertainty is large and we require further data reduction for confirmation.
143 citations
TL;DR: In this article, the authors investigated the climatology of the equatorial mass anomaly (EMA) in the thermosphere using 4 years of CHAMP measurements and revealed strong variation of the EMA with season and solar flux level.
Abstract: [1] The equatorial anomaly is an interesting and important feature of the Earth's thermosphere-ionosphere coupling in tropical regions. It is an anomalous latitudinal distribution found in both the ionized and unionized part of the atmosphere. Its equinox configuration consists of a minimum near the dip equator flanked by two maxima on both sides. The ionospheric side of this anomaly, often referred to as the equatorial ionization anomaly (EIA), has long been recognized since the 1930s. However, its thermospheric counterpart was only to be glimpsed by the Dynamic Explorer 2 satellite in the 1970s. A global picture of it has been rather recently revealed by the CHAMP satellite in 2005. In this paper we complement previous studies by investigating the climatology of the equatorial mass anomaly (EMA) in the thermosphere using 4 years of CHAMP measurements. Our analysis has revealed strong variation of the EMA with season and solar flux level. The EMA structure is most prominent around equinox, with a crest-to-trough ratio about 1.05 for F10.7 = 150. Near solstices, it is asymmetric about the dip equator. The density crest attains maximum 1–2 hours earlier and reaches higher values in the summer hemisphere than in the winter hemisphere. The density in EMA regions varies semiannually, with maxima near equinoxes. The latitudinal locations of the EMA crests undergo a seasonal variation, obviously following the movement of the subsolar point. The EMA structure has also been found to become more pronounced at higher solar flux levels. Both the location and magnitude of the EMA crests closely follow those of the EIA in corresponding seasons and solar flux levels, hence demonstrating strong plasma-neutral interaction. Furthermore, two seasonal asymmetries clearly present in the globally averaged density, with the density in March/December being ∼15–20% higher than that in September/June.
119 citations
TL;DR: In this paper, Sonnerup et al. used the deHoffmann-Teller frame of reference to analyze magnetopause crossings by the AMPTE/IRM spacecraft, which exhibited large shear in the magnetic field.
Abstract: A number of magnetopause crossings by the AMPTE/IRM spacecraft, which exhibited large shear in the magnetic field, were studied recently by Paschmann et al. (1986) in order to assess the presence or absence of reconnection processes at the dayside magnetopause. In the present article, eleven of these crossings are reanalyzed by use of methods described by Sonnerup et al. (1987). Although only eight of the cases lend themselves to this more detailed analysis procedure, the results derived when the analysis is successful are considerably more detailed and quantitative than those obtained previously. Four such cases are documented in detail in the paper. For these events it was possible to derive accurate vectors, n, normal to the magnetopause. It was also possible to place bounds on the values of average magnetic field and flow velocity components along n as well as on the average electric field tangential to the magnetopause and on the velocity of the magnetopause itself along n and therefore on the magnetopause thickness. It is particularly noteworthy that in two crossings (on September 4, 1984) it was possible to place lower bounds of 2.8 mV/m and 0.5 mV/m on the tangential (reconnection) electric field and corresponding lower bounds on normal flow and field. In a third crossing (September 8, 1984), located about 1 RE north of the subsolar point, evidence is deduced, indicating that the observed plasma acceleration along the magnetopause had a large temporal component, caused by a tangential gradient in the total pressure, in addition to the usual convective component caused by the Maxwell shear stresses. Finally, in the fourth crossing (October 19, 1984), large magnetic shear was present but, in agreement with a conclusion reached by Paschmann et al. (1986), the analysis shows that no evidence for reconnection was present and that the magnetopause had the properties of a tangential discontinuity. This case provides convincing evidence that the mere existence of a deHoffmann-Teller frame of reference in which the flow is field aligned (an excellent fit of the data to such a frame could be performed in this case) does not guarantee that the magnetopause is a rotational discontinuity. The remaining events analyzed by use of the new procedure demonstrate that the methodology sometimes fails, or yields unconvincing results, even when there is considerable evidence, in the form of accelerated plasma flows, that some form of reconnection is in progress.
107 citations
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Performance Metrics
| Year | Papers |
|---|---|
| 2021 | 2 |
| 2020 | 4 |
| 2019 | 7 |
| 2018 | 3 |
| 2017 | 4 |
| 2016 | 7 |