Browsing by Author "Ruohoniemi, John M."

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    Geospace Plume and Its Impact on Dayside Magnetopause Reconnection Rate
    Zou, Ying; Walsh, Brian M.; Shi, Xueling; Lyons, Larry; Liu, Jiang; Angelopoulos, Vassilis; Ruohoniemi, John M.; Coster, Anthea J.; Henderson, Michael G. (2021-06)
    The role a geospace plume in influencing the efficiency of magnetopause reconnection is an open question with two contrasting theories being debated. A local-control theory suggests that a plume decreases both local and global reconnection rates, whereas a global-control theory argues that the global reconnection rate is controlled by the solar wind rather than local physics. Observationally, limited numbers of point measurements from spacecraft cannot reveal whether a local change affects the global reconnection. A distributed observatory is hence needed to assess the validity of the two theories. We use THEMIS and Los Alamos National Laboratory spacecraft to identify the occurrence of a geospace plume and its contact with the magnetopause. Global evolution and morphology of the plume is traced using GPS measurements. SuperDARN is then used to monitor the distribution and the strength of dayside reconnection. Two storm-time geospace plume events are examined and show that as the plume contacts the magnetopause, the efficiency of reconnection decreases at the contact longitude. The amount of local decrease is 81% and 68% for the two events, and both values are consistent with the mass loading effect of the plume if the plume's atomic mass is similar to 4 amu. Reconnection in the surrounding is enhanced, and when the solar wind driving is stable, little variation is seen in the cross polar cap potential. This study illuminates a pathway to resolve the role of cold dense plasma on solar wind-magnetosphere coupling, and the observations suggest that plumes redistribute magnetopause reconnection activity without changing the global strength substantially. Plain Language Summary A variety of magnetospheric plasma populations exist at the interface where the solar wind encounters the magnetosphere, and they can impact the efficiency of the energy transfer from the solar wind to the magnetosphere. One population of particular interest is geospace plumes, which are plumes of cold dense plasma of ionospheric origin drifting sunward toward the magnetopause during enhanced geomagnetic activity. Plumes often have a density much higher than the other magnetospheric populations and can therefore mass load the dayside magnetopause slowing down magnetic reconnection. However, whether reconnection is slowed at a local or global scale is under debate. Observationally, point measurements from spacecraft cannot reveal whether a local change affects the global reconnection and a distributed observatory is hence needed. In this study we strategically coordinate measurements made by THEMIS and Los Alamos National Laboratory spacecraft, GPS network, and SuperDARN to investigate the effect of plumes on reconnection. Our results suggest that plumes decrease the local reconnection rate at the plume longitude and increase the reconnection rate in regions adjacent to the plume. When the solar wind is stable, the global reconnection remains unchanged. Such observations illuminate a pathway to resolve the role of cold dense plasma on solar wind-magnetosphere coupling.
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    Observations and Modeling Studies of Solar Eclipse Effects on Oblique High Frequency Radio Propagation
    Moses, M. L.; Kordella, L. J.; Earle, Gregory D.; Drob, Douglas P.; Huba, J. D.; Ruohoniemi, John M.; Shepherd, Simon G.; Sivakumar, V (2021-03)
    The total solar eclipse over the continental United States on 21 August 2017 offered a unique opportunity to study the dependence of the ionospheric density and morphology on incident solar radiation at different local times. The Super Dual Auroral Radar Network (SuperDARN) radars in Christmas Valley, Oregon, and Fort Hays, Kansas, are located slightly southward of the line of totality; they both made measurements of the eclipsed ionosphere. The received power of backscattered signal decreases during the eclipse, and the slant ranges from the westward looking radar beams initially increase and then decrease after totality. The time scales over which these changes occur at each site differ significantly from one another. For Christmas Valley the propagation changes are fairly symmetric in time, with the largest slant ranges and smallest power return occurring coincident with the closest approach of totality to the radar. The Fort Hays signature is less symmetric. In order to investigate the underlying processes governing the ionospheric eclipse response, we use a ray-tracing code to simulate SuperDARN data in conjunction with different eclipsed ionosphere models. In particular, we quantify the effect of the neutral wind velocity on the simulated data by testing the effect of adding/removing various neutral wind vector components. The results indicate that variations in meridional winds have a greater impact on the modeled ionospheric eclipse response than do variations in zonal winds. The geomagnetic field geometry and the line-of-sight angle from each site to the Sun appear to be important factors that influence the ionospheric eclipse response.
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    Simultaneous Development of Multiple Auroral Substorms: Double Auroral Bulge Formation
    Ohtani, S.; Gjerloev, J. W.; McWilliams, K. A.; Ruohoniemi, John M.; Frey, H. U. (2021-05)
    The expansion phase of auroral substorms is characterized by the formation of an auroral bulge, and it is generally considered that a single bulge forms following each substorm onset. However, we find that occasionally two auroral intensifications takes place close in time but apart in space leading to the formation of double auroral bulges, which later merge into one large bulge. We report three such events. In those events the westward auroral electrojet intensified in each auroral bulge, and geosynchronous magnetic field dipolarized in the same sector. It appears that two substorms took place simultaneously, and each substorm was accompanied by the formation of its own substorm current wedge system. This finding strongly suggests that the initiation of auroral substorms is a local process, and there is no global reference frame for their development. For example, ideas such as (i) the auroralbreakup takes place in the vicinity of the Harang reversal and (ii) the westward traveling surge maps to the interface between the plasma sheet and low-latitude boundary layer, do not necessarily hold for every substorm. Even if those ideas may be suggestive of causal magnetospheric processes, the reference structures themselves are probably not essential. It is also found that despite the formation of two distinct auroral bulges, the overall magnetosphere-ionosphere current system is represented by one globally coherent system, and we suggest that its structure is determined by the relative intensities and locations of the two substorm current wedges that correspond to the individual auroral bulges.