SH23C-01
Validation of a Global 3D-MHD Model for the Synoptic Solar Wind
We developed a global 3D MHD model for the ambient solar. The model is driven by SOHO MDI magnetogram data, and is constrained by the empirical Wang-Sheeley-Arge (WSA) model. The WSA provides the distribution of the asymptotic solar wind speed; therefore it also provides the distribution of the kinetic energy. Assuming that far from the Sun the total energy is dominated by the energy of the bulk motion and assuming the conservation of the Bernoulli integral, we can trace the total energy along a magnetic field line to the solar surface. Since the gravitational energy on the surface is known, and the kinetic energy is negligible, we can obtain the distribution of the polytropic index on the surface of the Sun. By interpolating the value of the polytropic index to a spherically constant value of 1.1 at the source surface and to 1.5 at 5 Rs, we use this spatial distribution of the polytropic index in the energy equation to get a self-consistent, steady state MHD solution for the ambient, synoptic solar wind. We present a long-term comparison of the model results with observations at 1AU.
SH23C-02
Understanding the relationship between photospheric magnetic field observations and in situ observations of the interplanetary magnetic field
Understanding the Sun's open flux and its variability during the course of the solar cycle is important for a number of reasons. For example, recent claims that it has increased significantly over the last century may have had significant space- and even terrestrial-weather consequences. A key relationship in understanding this evolution lies between the observed photospheric magnetic field and the open flux measured in situ by spacecraft. Global potential field source surface (PFSS) and MHD models can be used to address this relationship; however, several issues make this a difficult task. First, there is controversy about how to convert un-calibrated magnetogram measurements at some solar observatories into radial magnetic fields (which are the primary input into most numerical models). Second, it is not clear what contribution coronal mass ejections (CMEs) and other transient phenomena make to the observed open flux in interplanetary space. Third, it is difficult to assess what errors the different models introduce. In this study we investigate the relationship between solar observations of the photospheric magnetic field and in situ measurements by addressing each issue systematically. Here, we focus on deriving the best estimate of the photospheric magnetic field by inter-calibrating data from a number of solar observatories and assessing the sensitivity of the models to these inputs.
SH23C-03
Balancing the Global Heliospheric Flux Budget
Interplanetary coronal mass ejections (ICMEs) have long been identified in situ by the presence of counterstreaming suprathermal electrons (CSEs) observed in situ. The presence of CSEs has been interpreted as indicating that the field lines along which they stream remain connected to the Sun as far away as 5 AU. In recent years, studies have shown that the degree of openness of ICMEs does not change much between 1 AU and 5 AU, hence indicating that the rate of opening cannot be easily described by simple models previously proposed, and motivated by solar observations. The degree of openness of the fields at 1 AU and 5 AU implies that the fields open slowly, raising the issue of a flux catastrophe: an unbounded buildup of flux in the outer heliosphere. Recently, Owens and Crooker [2006] (OC06) derived reconnection times for the decay of closed fields in ICMEs to have timescales of ~50 days. In this paper, we argue that reconnection timescales must be on the order of 10 days or less, in order to prevent an overwhelming buildup of magnetic flux in the heliosphere. In order to facilitate comparisons with OC06 we approach this investigation by starting with a simplified view where CMEs are released at uniform time intervals with uniform strengths. We then evolve our model to include observed ICME rates and representative ICME strengths. We conclude that in order to match observed variations in the heliospheric flux over the solar cycle, reconnection times must be under 10 days, which is much lower than previously derived by OC06. These results directly contradict the findings from CSEs and raise important questions regarding their nature and our understanding of the interaction of the solar magnetic fields as they expand into the heliosphere.
SH23C-04
Reassessment of Arguments For and Against Open-Flux Conservation in the Heliosphere
Whether or not open magnetic flux from the Sun is conserved in the heliosphere has become a key issue for distinguishing between models of the solar-cycle reversal of magnetic polarity. An argument used in support of open-flux conservation is that dropouts of suprathermal electrons (EDs) that presumably signal a loss of open flux through disconnection from the Sun are rarely observed in the solar wind. On the other hand, coronagraph observations of inflows have been used as support for the argument that disconnection is as common as that predicted by potential field source surface (PFSS) models, which do not conserve open flux. We suggest that neither of these arguments is valid. The argument that rare EDs indicate negligible disconnection leaves open the question of what ED occurrence rate is required to balance the closed flux introduced by coronal mass ejections (CMEs). Recent modeling using observed CME rates indicates that the ED rate should be much lower than expected, consistent with the most recent observations of EDs using high-time-resolution data and thus consistent with no conservation of open flux. Countering this view, however, is the realization that EDs can also be caused by interchange reconnection at the Sun, which conserves open flux. Interchange reconnection can cancel the flux contribution from CMEs by opening them. If it occurs at the foot of a loop that has expanded far out into the heliosphere, the loop will appear to be an open field line at 1 AU, as required for disconnection, because the sunward electrons in the counterstreaming beam will have disappeared owing to scattering along the increasing path length. Regarding the argument concerning coronal inflows, these also can be caused by interchange reconnection with loops that have already expanded into the heliosphere. Moreover, what PFSS models count as disconnection could as well be interchange reconnection with loops that have expanded beyond the source surface. We conclude that observations and models can be viewed as fully consistent with conservation of open flux, attractive for its simplicity; but they cannot exclude the opposing view that the closed flux introduced by CMEs is somehow balanced by disconnection.
SH23C-05
Encounter of Ulysses with Comet McNaught
Comet C/2006 P1 McNaught was the brightest comet observed from Earth in the last 40 years. On February 3, 2007 the comet at a heliocentric distance of ~0.71 AU was nearly radially aligned with the Ulysses spacecraft at a heliocentric distance of ~2.40 AU and at 79 Deg south heliographic latitude. Thus, during a ~4.5-day interval (February 5-9) Ulysses encountered the tail region of this spectacular comet, the region of disturbance in the solar wind produced by the comet being nearly 10 Mkm wide at 2.4 AU. During the encounter the speed of the solar wind dropped from ~750 km/s to a minimum of 360 km/s, the proton density dropped by more than 2 orders of magnitude, and the proton temperature increased from ~1.5x105 to ~4x105 K, while simultaneously very large fluxes of cometary molecular and singly and doubly charged atomic ions were detected. The slowing, depletion and heating of the solar wind proton beam was a result of charge exchange with neutral atoms and molecules in the cometary atmosphere and with the pickup up by the wind of the newly-born cometary ions. Although no shocks were observed during the encounter, the magnetic field strength was slightly enhanced in broad regions at the leading and trailing edges of the tail of the comet and was generally weaker than in the unobstructed solar wind within the heart of the region of interaction. For most of the encounter, the magnetic field direction was nearly radially inward, and thus reversed from its normal outward direction in the southern polar hemisphere at this phase of the present solar cycle. There were, however, shorter periods when the field pointed nearly radially outward, indicating a filamentary structure of the comet tail. One of the intervals of radially outward field coincided with the interval of minimum flow speed and maximum flux of picked up O+ ions.
SH23C-06
Influence of the Magnetic Cloud Propagation on the Local Shape of the Heliospheric Current Sheet
The relationship between Magnetic Clouds (MC) and the Heliospheric Current Sheet (HCS) is not completely understood. The presence of current sheets close to the MCs passage has been widely observed. The main problem is to establish if those currents are originated along the MC travel through the solar wind or, in the opposite hand, they already existed previously to the MC arrival to the observation point and therefore, they could be directly connected with structures in the solar corona. In order to analyze this problem we selected twenty five MCs detected by WIND instruments that were accompanied by a leading or a following HCS crossing (or both) during the period 1995-2002. The procedure has been the following: Once a HCS crossing was confirmed, we have used analytical models for Magnetic Clouds and HCS local structure in order to establish the relative orientation between the MC axis and the normal vector to the HCS plane. Afterwards, we have compared these results with the Neutral Line inclination obtained from the Surface Source models. The preliminary results suggest a strong distortion of the local inclination of the HCS and a slight relationship between the MC speed and the relative orientation between the MC and the HCS.
SH23C-07
Heating of Coronal Loops: Weak MHD Turbulence and Scaling Laws.
We have investigated the nonlinear dynamics of the Parker Scenario for coronal heating through Reduced MHD long-time high-resolution simulations. A coronal loop is modeled as an elongated Cartesian box embedded in uniform and strong axial magnetic field, whose footpoints are convected by motions at the top and bottom planes, mimicking the photospere. We unambiguously identify MHD anisotropic turbulence as the physical mechanism responsible for the transport of energy from the large scales, where energy is injected by photosperic motions, to the small scales, where it is dissipated. This allows us to give analytical estimates of the heating rate for coronal loops as a function of the loop parameters, i.e. lengths, Alfvén velocity, forcing intensity. The predicted heating rate is within the lower range of observed active region and quiet-Sun coronal energy losses.
SH23C-08
CUNY Sun-Earth Research, Space Climate
Faculty and students at Queensborough Community College and Medgar Evers College of the City University of New York (CUNY) have, over several years now, employed simple software familiar to most undergraduate students to perform useful calculations, including statistical analyses, regarding various geophysical phenomena. Topics have included Space Weather, Interplanetary Magnetic Field (IMF) direction and strength fluctuations, geomagnetic and ionospheric responses to solar flares, and Coronal Mass Ejection (CME) events. Our statistical analyses have utilized second-order measures of fluctuation of the IMF strength, especially what we now call the Cheung number: the number of times that the value of Sigma-B, as provided by the ACE (Advanced Composition Explorer) data, has exceeded 0.5nT during a 6 hour interval. We have also utilized the Higuchi fractal dimension of various somewhat random fluctuations, including Sigma-B and the brightness or strength of adjacent pixels or data points in somewhat random data sequences in time or spatial dimension, including IMF fluctuations and SOHO (Solar Heliographic Observer) images of the Sun. These we have correlated with each other and with such variables as SEP (Solar Energetic Particle) peak flux, TEC (Total Electron Content) of the ionosphere, and Dst (Disturbance storm-time) in the geomagnetic field. Recent results indicate that the IMF fluctuation measures are well correlated with the SEP peak flux, the Dst, and TEC. Higuchi fractal analysis of SOHO photospheric ultraviolet brightness indicates, consistent with concomitant increased chaos or randomness of photospheric brightness, an increased likelihood of solar flare events or CME affecting interplanetary space and the earth's magnetosphere/ionosphere/atmosphere.