Impact of the altitudinal Joule heating distribution on the thermosphere

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95601/1/jgra20978.pd

Modelling of composition changes during F-region storms: a reassessment

A recalculation of the global changes of thermospheric gas composition, resulting from strong heat inputs in the auroral ovals, shows that (contrary to some previous suggestions) widespread increases of mean molecular mass are produced at mid-latitudes, in summer and at equinox. Decreases of mean molecular mass occur at mid-latitudes in winter. Similar results are given by both the `UCL' and `NCAR TIGCM' three-dimensional models. The computed composition changes now seem consistent with the local time and seasonal response observed by satellites, and can broadly account for `negative storm effects' in the ionospheric F2-layer at mid-latitudes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29311/1/0000375.pd

A comparison of wind observations of the upper thermosphere from the dynamics explorer satellite with the predictions of a global time-dependent model

Seven polar passes of the NASA Dynamics Explorer 2 (DE-2) satellite during October and early December 1981 have been used to examine the high-latitude circulation in the upper thermosphere. Vector winds along the satellite track are derived by appropriate merging of the data from the remote-sensing Fabry-Perot interferometer (meridional wind) and the in situ wind and temperature spectrometer (zonal wind) and are compared with the predictions of a three-dimensional, time-dependent, global model of the thermosphere. Major features of the experimental winds, such as the mean day to night circulation caused by solar u.v. and e.u.v. heating, augmented by magnetospheric processes at high latitude and the sharp boundaries and flow reversals imposed on thermospheric winds by momentum transfer (ion drag) from the magnetosphere, are qualitatively explained by a version of the global model using a semi-empirical global model of polar electric fields (Volland Model 2 or Heppner Model A) and a model of global electron density which excludes the effects of high-latitude geomagnetic processes. A second version of the global dynamic model includes a theoretical model of the high-latitude ionosphere which is self-consistent and reflects the enhancement of ionization due to magnetospheric phenomena acting in addition to solar e.u.v. photo-ionization, including the interactive processes which occur between ionization and high latitude ion convection and thermospheric winds. This second dynamical model shows an improved comparison with the structure and magnitude of polar cap and auroral oval winds at times of other than extremely low geomagnetic activity, when the first model appears a better match. An improved empirical description of the complex magnetospheric processes exciting the thermosphere in the vicinity of the dayside polar cusp and an empirical description of storm-time electric fields will be required for a quantitative explanation of the polar thermospheric winds during geomagnetic substorm events.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25079/1/0000510.pd

The westward thermospheric jet-stream of the evening auroral oval

One of the most consistent and often dramatic interactions between the high latitude ionosphere and the thermosphere occurs in the vicinity of the auroral oval in the afternoon and evening period. Ionospheric ions, convected sunward by the influence of the magnetospheric electric field, create a sunward jet-stream in the thermosphere, where wind speeds of up to 1 km s-1 can occur. This jet-stream is nearly always present in the middle and upper thermosphere (above 200 km altitude), even during periods of very low geomagnetic activity. However, the magnitude of the winds in the jet-stream, as well as its location and range in latitude, each depend on geomagnetic activity. On two occasions, jet-streams of extreme magnitude have been studied using simultaneous ground-based and satellite observations, probing both the latitudinal structure and the local time dependence. The observations have then been evaluated with the aid of simulations using a global, three-dimensional, time-dependent model of thermospheric dynamics including the effects of magnetospheric convection and particle precipitation. The extreme events, where sunward winds of above 800 ms-1 are generated at relatively low geomagnetic latitudes (60-70[deg]) require a greatly expanded auroral oval and large cross-polar cap electric field ( ~ 150 kV). These in turn are generated by a persistent strong Interplanetary Magnetic Field, with a large southward component. Global indices such as Kp are a relatively poor indicator of the magnitude and extent of the jet-stream winds.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25724/1/0000281.pd

A theoretical and empirical study of the response of the high latitude thermosphere to the sense of the "Y" component of the interplanetary magnetic field

The strength and direction of the Interplanetary Magnetic Field (IMF) controls the transfer of solar wind momentum and energy to the high latitude thermosphere in a direct fashion. The sense of " Y" component of the IMF (BY) creates a significant asymmetry of the magnetospheric convection pattern as mapped onto the high latitude thermosphere and ionosphere. The resulting response of the polar thermospheric winds during periods when BY is either positive or negative is quite distinct, with pronounced changes in the relative strength of thermospheric winds in the dusk-dawn parts of the polar cap and in the dawn part of the auroral oval. In a study of four periods when there was a clear signature of BY, observed by the ISEE-3 satellite, with observations of polar winds and electric fields from the Dynamics Explorer-2 satellite and with wind observations by a ground-based Fabry-Perot interferometer located in Kiruna, Northern Sweden, it is possible to explain features of the high latitude thermospheric circulation using three dimensional global models including BY dependent, asymmetric, polar convection fields. Ground-based Fabry-Perot interferometers often observe anomalously low zonal wind velocities in the (Northern) dawn auroral oval during periods of extremely high geomagnetic activity when BY is positive. Conversely, for BY negative, there is an early transition from westward to southward and eastward winds in the evening auroral oval (excluding the effects of auroral substorms), and extremely large eastward (sunward) winds may be driven in the auroral oval after magnetic midnight. These observations are matched by the observation of strong anti-sunward polar-cap wind jets from the DE-2 satellite, on the dusk side with BY negative, and on the dawn side with BY positive.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26311/1/0000396.pd

Validation of Ionospheric Specifications During Geomagnetic Storms: TEC and foF2 During the 2013 March Storm Event

To address challenges of assessing space weather modeling capabilities, the CommunityCoordinated Modeling Center is leading a newly establishedInternational Forum for Space WeatherModeling Capabilities Assessment. This paper presents preliminary results of validation of modeled foF2 (F2 layer critical frequency) and TEC (total electron content) during the first selected 2013 March storm event (17 March 2013). In this study, we used eight ionospheric models ranging from empirical to physics-based, coupled ionosphere-thermosphere and data assimilation models. The quantities we considered are TEC and foF2 changes and percentage changes compared to quiet time background, and the maximum and minimum percentage changes. In addition, we considered normalized percentage changes of TEC. We compared the modeled quantities with ground-based observations of vertical Global Navigation SatelliteSystem TEC (provided by Massachusetts Institute of Technology Haystack Observatory) and foF2 data (provided by Global Ionospheric Radio Observatory) at the 12 locations selected in middle latitudes of the American and European-African longitude sectors. To quantitatively evaluate the models’ performance, we calculated skill scores including correlation coefficient, root-mean square error (RMSE), ratio of the modeled to observed maximum percentage changes (yield), and timing error. Our study indicates that average RMSEs of foF2range from about 1 MHz to 1.5 MHz. The average RMSEs of TEC are between ~5 and ~10 TECU (1 TEC Unit= 1016el/m2). dfoF2[%] RMSEs are between 15% and 25%, which is smaller than RMSE of dTEC[%] ranging from30% to 60%. The performance of the models varies with the location and metrics considered

On the Sources of Cold and Dense Plasma in Plasmasphere Drainage Plumes

Previous observations have revealed that the ionospheric Storm Enhanced Density (SED) plumes are colocated with the cold, dense plumes observed at the dayside magnetopause. However, the origin of the cold, dense plume plasma is not well understood with multiple possible sources in the magnetosphere. Improving our understanding of these plasmaspheric plumes is crucial due to their impact on the dayside magnetic reconnection. We report that plumes were simultaneously observed in both the ionosphere and plasmasphere by TEC and geosynchronous spacecraft for the magnetic storms occurred in 2013 and 2015 on Mar 17. Moreover, in 2015, the plume was also observed by THEMIS spacecraft near the dayside magnetopause. Simulations using a physics-based model of the ionosphere and plasmasphere reproduced the observed plume colocation in the ionosphere and plasmasphere for both storms. Our results suggest that plasmaspheric plume was created by the enhanced convection transporting the plasma sunward that was peeled off from the outer plasmasphere, whereas the ionospheric plume plasma came from the density enhancement generated in the dayside subauroral ionosphere. These plumes were observed near the same closed field lines at the peak of the geomagnetic activity because the cold plasma motion in the ionosphere and plasmasphere is connected through the ExB drift motion. Furthermore, our results suggest that weaker storms transport more plasmaspheric materials toward the dayside/duskside magnetopause. However stronger storms may have a larger impact on the dayside reconnection because plasmaspheric plumes tend to be shifted to the noon MLT sector where dayside reconnections more likely to occur. &nbsp; &nbsp; Plain Language Summary The plasmasphere is the region of cold, relatively dense ionized gas (mostly protons and helium ions) that resides on the magnetic field lines close to the Earth. It is the upward extension of cold, dense Earth&rsquo;s ionospheric plasma as the ionosphere had filled the persistently &ldquo;closed&rdquo; flux tubes of plasmasphere. Enhanced convection plasma flow during solar storms peels the cold, dense plasma away from the outer plasmasphere to form a plume of plasma that moves sunward. Recently, the plasmaspheric cold, dense plasma has been found near where the Earth&rsquo;s magnetic field first contacts the solar wind, however, where the plasma originates from remains unclear. Understanding the plume plasma is very important because they could rearrange the magnetic topology by altering the rate of the magnetic reconnection, which determines how much solar wind energy gets into the Earth&rsquo;s magnetosphere. Here we report that in two solar storms in 2013 and 2015, plumes were observed simultaneously in both ionosphere and plasmasphere. Our study suggests that the observed plumes in the plasmasphere and ionosphere were mainly created by different mechanisms, but were observed near the same magnetic field lines at the peak of the solar storms.</p

Hemispheric asymmetries in thermospheric structure and dynamics

Hemispheric differences in the structure and dynamics of the polar regions of the thermosphere and ionosphere are caused by seasonal/latitudinal asymmetries of solar insolation acting in addition to the effects of the distinctive asymmetries of the main geomagnetic field. Viewing the earth from a satellite, the longitudinal and universal time variations of the thermosphere and ionosphere are far more spectacular in the southern polar thermosphere and ionosphere than in the northern polar regions. This is simply caused by the greater offset of the geomagnetic pole from the geographic pole in the southern hemisphere, and the associated geomagnetic forcing of the polar thermosphere and ionosphere. The southern polar cusp is a localised region of intense forcing with a diurnal migration in geographic latitude from 55° to the southern geographic pole. At times of major geomagnetic disturbances, the combination of the equatorward expansion of the auroral oval and the greater offset of the southern geomagnetic pole from the rotational pole causes dramatic effects in southern mid-latitude and near-equatorial regions. The northern auroral oval rarely expands within 40° of the equator (at mid-American longitudes). In the Australian sector, equivalent major geomagnetic disturbances may extend to a southern latitude of 30°. Wind, gravity wave and ionospheric perturbations originating in the auroral ovals decrease rapidly in amplitude with increasing distance from the source region. The greater frequency and amplitude of auroral-related disturbances reaching a particular mid- or low-latitude region in the southern hemisphere, particularly in the Australian sector is thus explained. A survey of recent satellite data will be presented, highlighted by simulations using a global theoretical model, to demonstrate the causes of these hemispheric asymmetries. Future complementary ground-based and space measurements can exploit the hemispheric asymmetries to clearly identify geomagnetic forcing mechanisms. The greater UT and latitudinal modulation in the southern hemisphere may be used to identify sub-auroral excitation of the thermosphere and ionosphere during major storms. The cause of the related negative F-region storm-time response is still not certain, even after 30 years of study