815 research outputs found
Flux-tube geometry and solar wind speed during an activity cycle
The solar wind speed at 1 AU shows variations in latitude and in time which
reflect the evolution of the global background magnetic field during the
activity cycle. It is commonly accepted that the terminal wind speed in a
magnetic flux-tube is anti-correlated with its expansion ratio, which motivated
the definition of widely-used semi-empirical scaling laws relating one to the
other. In practice, such scaling laws require ad-hoc corrections. A predictive
law based solely on physical principles is still missing. We test whether the
flux-tube expansion is the controlling factor of the wind speed at all phases
of the cycle and at all latitudes using a very large sample of wind-carrying
open magnetic flux-tubes. We furthermore search for additional physical
parameters based on the geometry of the coronal magnetic field which have an
influence on the terminal wind flow speed. We use MHD simulations of the corona
and wind coupled to a dynamo model to provide a large statistical ensemble of
open flux-tubes which we analyse conjointly in order to identify relations of
dependence between the wind speed and geometrical parameters of the flux-tubes
which are valid globally (for all latitudes and moments of the cycle). Our
study confirms that the terminal speed of the solar wind depends very strongly
on the geometry of the open magnetic flux-tubes through which it flows. The
total flux-tube expansion is more clearly anti-correlated with the wind speed
for fast rather than for slow wind flows, and effectively controls the
locations of these flows during solar minima. Overall, the actual asymptotic
wind speeds attained are also strongly dependent on field-line inclination and
magnetic field amplitude at the foot-points. We suggest ways of including these
parameters on future predictive scaling-laws for the solar wind speed.Comment: Accepted for publicaton on Astronomy & Astrophysic
Models and data analysis tools for the Solar Orbiter mission
Context. The Solar Orbiter spacecraft will be equipped with a wide range of remote-sensing (RS) and in-situ (IS) instruments to record novel and
unprecedented measurements of the solar atmosphere and the inner heliosphere. To take full advantage of these new datasets, tools and techniques
must be developed to ease multi-instrument and multi-spacecraft studies. In particular the currently inaccessible low solar corona below two solar
radii can only be observed remotely. Furthermore techniques must be used to retrieve coronal plasma properties in time and in three dimensional
(3D) space. Solar Orbiter will run complex observation campaigns that provide interesting opportunities to maximise the likelihood of linking IS
data to their source region near the Sun. Several RS instruments can be directed to specific targets situated on the solar disk just days before data
acquisition. To compare IS and RS, data we must improve our understanding of how heliospheric probes magnetically connect to the solar disk.
Aims. The aim of the present paper is to briefly review how the current modelling of the Sun and its atmosphere can support Solar Orbiter science.
We describe the results of a community-led effort by European Space Agency (ESA)’s Modelling and Data Analysis Working Group (MADAWG)
to develop different models, tools, and techniques deemed necessary to test different theories for the physical processes that may occur in the
solar plasma. The focus here is on the large scales and little is described with regards to kinetic processes. To exploit future IS and RS data fully,
many techniques have been adapted to model the evolving 3D solar magneto-plasma from the solar interior to the solar wind. A particular focus
in the paper is placed on techniques that can estimate how Solar Orbiter will connect magnetically through the complex coronal magnetic fields to
various photospheric and coronal features in support of spacecraft operations and future scientific studies.
Methods. Recent missions such as STEREO, provided great opportunities for RS, IS, and multi-spacecraft studies. We summarise the achievements
and highlight the challenges faced during these investigations, many of which motivated the Solar Orbiter mission. We present the new tools and
techniques developed by the MADAWG to support the science operations and the analysis of the data from the many instruments on Solar Orbiter.
Results. This article reviews current modelling and tool developments that ease the comparison of model results with RS and IS data made
available by current and upcoming missions. It also describes the modelling strategy to support the science operations and subsequent exploitation
of Solar Orbiter data in order to maximise the scientific output of the mission.
Conclusions. The on-going community effort presented in this paper has provided new models and tools necessary to support mission operations
as well as the science exploitation of the Solar Orbiter data. The tools and techniques will no doubt evolve significantly as we refine our procedure
and methodology during the first year of operations of this highly promising mission
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Excess open solar magnetic flux from satellite data: 1. Analysis of the third perihelion Ulysses pass
We use the third perihelion pass by the Ulysses spacecraft to illustrate and investigate the “flux excess” effect, whereby open solar flux estimates from spacecraft increase with increasing heliocentric distance. We analyze the potential effects of small-scale structure in the heliospheric field (giving fluctuations in the radial component on timescales smaller than 1 h) and kinematic time-of-flight effects of longitudinal structure in the solar wind flow. We show that the flux excess is explained by neither very small-scale structure (timescales 1 day) solar wind speed variations on the frozen-in heliospheric field. We show that averaging over an interval T (that is long enough to eliminate structure originating in the heliosphere yet small enough to avoid cancelling opposite polarity radial field that originates from genuine sector structure in the coronal source field) is only an approximately valid way of allowing for these effects and does not adequately explain or account for differences between the streamer belt and the polar coronal holes
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Excess open solar magnetic flux from satellite data: 2. A survey of kinematic effects
We investigate the “flux excess” effect, whereby open solar flux estimates from spacecraft increase with increasing heliocentric distance. We analyze the kinematic effect on these open solar flux estimates of large-scale longitudinal structure in the solar wind flow, with particular emphasis on correcting estimates made using data from near-Earth satellites. We show that scatter, but no net bias, is introduced by the kinematic “bunching effect” on sampling and that this is true for both compression and rarefaction regions. The observed flux excesses, as a function of heliocentric distance, are shown to be consistent with open solar flux estimates from solar magnetograms made using the potential field source surface method and are well explained by the kinematic effect of solar wind speed variations on the frozen-in heliospheric field. Applying this kinematic correction to the Omni-2 interplanetary data set shows that the open solar flux at solar minimum fell from an annual mean of 3.82 × 1016 Wb in 1987 to close to half that value (1.98 × 1016 Wb) in 2007, making the fall in the minimum value over the last two solar cycles considerably faster than the rise inferred from geomagnetic activity observations over four solar cycles in the first half of the 20th century
The magnetic connectivity of coronal shocks from behind-the-limb flares to the visible solar surface during -ray events
Context. The observation of >100 MeV {\gamma}-rays in the minutes to hours
following solar flares suggests that high-energy particles interacting in the
solar atmosphere can be stored and/or accelerated for long time periods. The
occasions when {\gamma}-rays are detected even when the solar eruptions
occurred beyond the solar limb as viewed from Earth provide favorable viewing
conditions for studying the role of coronal shocks driven by coronal mass
ejections (CMEs) in the acceleration of these particles.
Aims: In this paper, we investigate the spatial and temporal evolution of the
coronal shocks inferred from stereoscopic observations of behind-the-limb
flares to determine if they could be the source of the particles producing the
{\gamma}-rays.
Methods: We analyzed the CMEs and early formation of coronal shocks
associated with {\gamma}-ray events measured by the Fermi-Large Area Telescope
(LAT) from three eruptions behind the solar limb as viewed from Earth on 2013
Oct. 11, 2014 Jan. 06 and Sep. 01. We used a 3D triangulation technique, based
on remote-sensing observations to model the expansion of the CME shocks from
above the solar surface to the upper corona. Coupling the expansion model to
various models of the coronal magnetic field allowed us to derive the
time-dependent distribution of shock Mach numbers and the magnetic connection
of particles produced by the shock to the solar surface visible from Earth.
Results: The reconstructed shock fronts for the three events became
magnetically connected to the visible solar surface after the start of the
flare and just before the onset of the >100 MeV {\gamma}-ray emission. The
shock surface at these connections also exhibited supercritical Mach numbers
required for significant particle energization.
[...] (Abridged)Comment: 20 pages, 15 figures, version published in A&
Tracking Streamer Blobs Into the Heliosphere
In this paper, we use coronal and heliospheric images from the STEREO
spacecraft to track streamer blobs into the heliosphere and to observe them
being swept up and compressed by the fast wind from low-latitude coronal holes.
From an analysis of their elongation/time tracks, we discover a 'locus of
enhanced visibility' where neighboring blobs pass each other along the line of
sight and their corotating spiral is seen edge on. The detailed shape of this
locus accounts for a variety of east-west asymmetries and allows us to
recognize the spiral of blobs by its signatures in the STEREO images: In the
eastern view from STEREO-A, the leading edge of the spiral is visible as a
moving wavefront where foreground ejections overtake background ejections
against the sky and then fade. In the western view from STEREO-B, the leading
edge is only visible close to the Sun-spacecraft line where the radial path of
ejections nearly coincides with the line of sight. In this case, we can track
large-scale waves continuously back to the lower corona and see that they
originate as face-on blobs.Comment: 15 pages plus 11 figures; figure 6 shows the 'locus of enhanced
visibility', which we call 'the bean'. (accepted by ApJ 4/02/2010
Simultaneous interplanetary scintillation and Heliospheric Imager observations of a coronal mass ejection
We describe simultaneous Interplanetary Scintillation (IPS) and STEREO Heliospheric Imager (HI) observations of a coronal mass ejection (CME) on 16 May 2007. Strong CME signatures were present throughout the IPS observation. The IPS raypath lay within the field-of-view of HI-1 on STEREO-A and comparison of the observations shows that the IPS measurements came from a region within a faint CME front observed by HI-1A. This front may represent the merging of two converging CMEs. Plane-of-sky velocity estimates based on time-height plots of the two converging CME structures were 325 kms?1 and 550 kms?1 for the leading and trailing fronts respectively. The plane-of-sky velocities determined from IPS ranged from 420 ± 10 kms?1 to 520 ± 20 kms?1. IPS results reveal the presence of micro-structure within the CME front which may represent interaction between the two separate CME events. This is the first time that it has been possible to interpret IPS observations of small-scale structure within an interplanetary CME in terms of the global structure of the event
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Oscillations in the open solar magnetic flux with a period of 1.68 years: imprint on galactic cosmic rays and implications for heliospheric shielding
An understanding of how the heliosphere modulates galactic cosmic ray (GCR) fluxes and spectra is important, not only for studies of their origin, acceleration and propagation in our galaxy, but also for predicting their effects (on technology and on the Earth’s environment and organisms) and for interpreting abundances of cosmogenic isotopes in meteorites and terrestrial reservoirs. In contrast to the early interplanetary measurements, there is growing evidence for a dominant role in GCR shielding of the total open magnetic flux, which emerges from the solar atmosphere and enters the heliosphere. In this paper, we relate a strong 1.68- year oscillation in GCR fluxes to a corresponding oscillation in the open solar magnetic flux and infer cosmic-ray propagation paths confirming the predictions of theories in which drift is important in modulating the cosmic ray flux
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Effects of solar wind magnetosphere coupling recorded at different geomagnetic latitudes: Separation of directly-driven and storage/release systems
The effect on geomagnetic activity of solar wind speed, compared with that of the strength of the interplanetary magnetic field, differs with geomagnetic latitude. In this study we construct a new index based on monthly standard deviations in the H-component of the geomagnetic field for all geomagnetic latitudes. We demonstrate that for this index the response at auroral regions correlates best with interplanetary coupling functions which include the solar wind speed while mid- and low-latitude regions respond to variations in the interplanetary magnetic field strength. These results are used to isolate the responsible geomagnetic current systems
Signatures of Interchange Reconnection: STEREO, ACE and Hinode Observations Combined
Combining STEREO, ACE and Hinode observations has presented an opportunity to
follow a filament eruption and coronal mass ejection (CME) on the 17th of
October 2007 from an active region (AR) inside a coronal hole (CH) into the
heliosphere. This particular combination of `open' and closed magnetic
topologies provides an ideal scenario for interchange reconnection to take
place. With Hinode and STEREO data we were able to identify the emergence time
and type of structure seen in the in-situ data four days later. On the 21st,
ACE observed in-situ the passage of an ICME with `open' magnetic topology. The
magnetic field configuration of the source, a mature AR located inside an
equatorial CH, has important implications for the solar and interplanetary
signatures of the eruption. We interpret the formation of an `anemone'
structure of the erupting AR and the passage in-situ of the ICME being
disconnected at one leg, as manifested by uni-directional suprathermal electron
flux in the ICME, to be a direct result of interchange reconnection between
closed loops of the CME originating from the AR and `open' field lines of the
surrounding CH.Comment: 13 pages, 13 figures, accepted Annales Geophysica
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