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  • 1
    Electronic Resource
    Electronic Resource
    Effect of areal expansion and coronal heating on the solar wind (1994)
    Springer
    Space science reviews 70 (1994), S. 387-390 
    Details
    ISSN: 1572-9672
    Keywords: Solar Physics ; Solar Corona ; Solar Wind
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Empirical studies have shown that the solar wind speed at Earth is inversely correlated with the areal expansion rate of magnetic flux tubes near the Sun. Recent model calculations that include a self-consistent determination of the coronal temperature allow one to understand the physical basis of this relationship; they also suggest why the solar wind mass flux is relatively constant.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00777897
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    Paper (German National Licenses)
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  • 2
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    Comparing Ulysses wind speed with coronal flux-tube expansion factor (1995)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: We have been comparing measurements of solar wind speed at the Ulysses spacecraft with coronal flux-tube expansion rates, derived from photospheric field measurements using a current-free coronal model. The large-scale patterns of derived speed have continued to reproduce the observed patterns from launch through south polar passage to the present 40S latitude of the spacecraft. The fastest non-transient wind speeds of approx. 860 km/s were encountered at midlatitudes en route to the south pole, rather than during polar passage when the peak speeds were approx. 820 km/s. Although this result is in qualitative agreement with the idea that the wind speed is controlled by the coronal flux-tube expansion rate, the 40 km/s difference is significantly smaller than the 100-150 km/s difference based on our in-ecliptic calibration. This paper will summarize our attempts to resolve this discrepancy and will show the observational status of our coronal/interplanetary comparison at the time of the meeting.
    Keywords: Solar Physics
    Type: ; 63
    Format: text
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  • 3
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    Source regions of the solar wind (1995)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: Using Skylab XUV data, we examine some properties of the source regions of the solar wind. In particular, we discuss the physical nature of polar plumes and their relationship to the polar wind, the nature of the source regions of the slow solar wind, and the relationship between abundance anomalies (the FIP effect) determined from the Skylab data and the sources of fast and slow wind.
    Keywords: Solar Physics
    Type: International Solar Wind 8 Conference; 31; NASA-CR-199940
    Format: text
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  • 4
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    MMS Observation of Inverse Energy Dispersion in Shock Drift Accelerated Ions (2017)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The four Magnetospheric Multiscale (MMS) spacecraft observed a 1 min burst of energetic ions (501000 keV) in the region upstream from the subsolar quasi-perpendicular bow shock on 6 December 2015. The composition, flux levels, and spectral indices of these energetic protons, helium, and oxygen ions greatly resemble those seen in the outer magnetosphere earlier while MMS crossed the magnetopause and differ significantly from those simultaneously observed far upstream by Advanced Composition Explorer (ACE). However, the event cannot be explained solely in terms of leakage from the magnetosphere. The strongly southward orientation of the interplanetary magnetic field (IMF) lines at the time of the event precludes any connection to the magnetosphere. This point is confirmed by the presence of energetic electrons, known to occur on magnetic field lines that graze the bow shock rather than connect to the magnetosphere. We suggest that the ions gradient drifted out of the nearby quasi-parallel foreshock and into the quasi-perpendicular bow shock. Each of the ion species exhibited an inverse energy dispersion. As predicted by models for shock drift acceleration, the energies of the ions increased as (sub Bn), the angle between the IMF and the shock normal, increased. Finally, we note that a similar event was observed a few minutes later in the subsolar magnetosheath, indicating that such events can be swept downstream of the bow shock.
    Keywords: Solar Physics
    Type: GSFC-E-DAA-TN64900 , GSFC-E-DAA-TN63349 , Journal of Geophysical Research: Space Physics (ISSN 2169-9402) (e-ISSN 2169-9380); 122; 3; 3232-3246
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  • 5
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    Converging Supergranular Flows and the Formation of Coronal Plumes (2016)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Earlier studies have suggested that coronal plumes are energized by magnetic reconnection between unipolar flux concentrations and nearby bipoles, even though magnetograms sometimes show very little minority-polarity flux near the footpoints of plumes. Here we use high-resolution extreme-ultraviolet (EUV) images and magnetograms from the Solar Dynamics Observatory (SDO) to clarify the relationship between plume emission and the underlying photospheric field. We find that plumes form where unipolar network elements inside coronal holes converge to form dense clumps, and fade as the clumps disperse again. The converging flows also carry internetwork fields of both polarities. Although the minority-polarity flux is sometimes barely visible in the magnetograms, the corresponding EUV images almost invariably show loop-like features in the core of the plumes, with the fine structure changing on timescales of minutes or less. We conclude that the SDO observations are consistent with a model in which plume emission originates from interchange reconnection in converging flows, with the plume lifetime being determined by the approximately 1-day evolutionary timescale of the supergranular network. Furthermore, the presence of large EUV bright points and/or ephemeral regions is not a necessary precondition for the formation of plumes, which can be energized even by the weak, mixed-polarity internetwork fields swept up by converging flows.
    Keywords: Solar Physics
    Type: GSFC-E-DAA-TN33266 , The Astrophysical Journal; 818; 2; 203
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  • 6
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    Evidence for Two Separate Heliospheric Current Sheets of Cylindrical Shape During Mid-2012 (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: During the reversal of the Sun's polar fields at sunspot maximum, outward extrapolations of magnetograph measurements often predict the presence of two or more current sheets extending into the interplanetary medium, instead of the single heliospheric current sheet (HCS) that forms the basis of the standard 'ballerina skirt' picture. By comparing potential-field source-surface models of the coronal streamer belt with white-light coronagraph observations, we deduce that the HCS was split into two distinct structures with circular cross sections during mid-2012. These cylindrical current sheets were centered near the heliographic equator and separated in longitude by roughly 180 deg; a corresponding four-sector polarity pattern was observed at Earth. Each cylinder enclosed a negative-polarity coronal hole that was identifiable in extreme ultraviolet images and gave rise to a high-speed stream. The two current sheet systems are shown to be a result of the dominance of the Sun's nonaxisymmetric quadrupole component, as the axial dipole field was undergoing its reversal during solar cycle 24.
    Keywords: Solar Physics
    Type: GSFC-E-DAA-TN22678 , The Astrophysical Journal; 780; 1; 103
    Format: application/pdf
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  • 7
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    Slow Solar Wind: Observations and Modeling (2016)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: While it is certain that the fast solar wind originates from coronal holes, where and how the slow solar wind (SSW) is formed remains an outstanding question in solar physics even in the post-SOHO era. The quest for the SSW origin forms a major objective for the planned future missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless, results from spacecraft data, combined with theoretical modeling, have helped to investigate many aspects of the SSW. Fundamental physical properties of the coronal plasma have been derived from spectroscopic and imaging remote-sensing data and in situ data, and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the SSW. Advanced models of the SSW in coronal streamers and other structures have been developed using 3D MHD and multi-fluid equations.
    Keywords: Solar Physics
    Type: GSFC-E-DAA-TN40692 , Space Science Reviews (ISSN 0038-6308) (e-ISSN 1572-9672); 201; 1; 55-108
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  • 8
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    Evidence for Two Separate but Interlaced Components of the Chromospheric Magnetic Field (2012)
    In:  CASI
    Details
    Publication Date: 2019-07-19
    Description: Chromospheric fibrils are generally thought to trace out horizontal magnetic fields that fan out from flux concentrations in the photosphere. A high-resolution (0.2") image taken in the core of the Ca IJ854.2 nm line shows the dark fibrils within an active region remnant as fine, loop-like features that are aligned parallel to each other and have lengths on the order of a supergranular diameter (approx.30 Mm). Comparison with a line-of-sight magnetogram confirms that the fibrils are centered above intranetwork areas, with one end rooted just inside the neighboring plage or strong unipolar network but the other endpoint less clearly defined. Focusing on a particular arcade-like structure lying entirely on one side of a filament channel (large-scale polarity inversion), we find that the total amount of positive-polarity flux underlying this "fibril arcade' is 50 times greater than the total amount of negative-polarity flux. Thus, if the fibrils represent closed loops, they must consist of very weak fields (in terms of flux density), which are interpenetrated by a more vertical field that contains most of the flux. This surprising result suggests that the fibrils in unipolar regions connect the network to the nearby intranetwork flux, while the bulk of the network flux is diverted upward into the corona and connects to remote regions of the opposite polarity. We conclude that the chromospheric field near the edge of the network has an interlaced structure resembling that in sunspot penumbrae, with the fibrils representing the low-lying horizontal flux that remains trapped within the highly nonpotential chromospheric layer.
    Keywords: Solar Physics
    Type: GSFC.ABS.01268.2012 , 220th Meeting of the American Astronomical Society (AAS); Jun 10, 2012 - Jun 14, 2012; Anchorage, AK; United States
    Format: application/pdf
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