SH24A-01

Towards a Consensus View of the Heliospheric Magnetic Field Strength Since 1900

Cliver, E W Edward.Cliver@hanscom.af.mil, Air Force Research Laboratory, Space Vehicles Directorate, Hanscom AFB, MA 01731-3010, United States
* Svalgaard, L leif@leif.org, Stanford University, HEPL, Stanford, CA 94305-4085, United States

McCracken (2007) inverted the galactic cosmic ray record for the interval 1428-2005 to estimate annual averages of the heliomagnetic field (HMF) near Earth during this interval. Quoting from his abstract, "There is good agreement with the results obtained by others using two independent methodologies based upon the sunspot [Solanki et al., 2002] and geomagnetic [Lockwood et al., 1999] records … There is disagreement with another method based on the geomagnetic record [Svalgaard and Cliver, 2005] that needs to be resolved." We address the reported disagreement of our long-term reconstruction of the HMF strength with that obtained in the other three studies. We show that a recent reconstruction of the HMF by Rouillard, Lockwood, and Finch [2007] agrees much more closely with that of Svalgaard and Cliver than that of Lockwood et al. to the point where a consensus seems to be reached. We suggest that the discrepancy between McCracken's cosmic-ray-based HMF reconstruction and those based on geomagnetic data originates in the Forbush and Neher ionization chamber data (1933-1957) used to bridge the time gap between the 10Be time series (1428-1930) and the Climax neutron monitor record (1951-present).

SH24A-02

The solar cycle, solar wind and heliospheric magnetic field: Ulysses latitude scans at two solar minima

Marsden, R Richard.Marsden@esa.int, ESA/ESTEC, P.O.Box 299, Noordwijk, 2200 AG, Netherlands
* Smith, E J edward.j.smith@jpl.nasa.gov, Jet Propulsion Laboratory, 4800 Oak Grove Dr, Pasadena, CA 91109, United States
McComas, D J DMcComas@swri.edu, Southwest Research Institute, 6220 Culebra Rd, San Antonio, TX 78238, United States
Issautier, K karine.issautier@obspm.fr, Observatoire de Paris, 5 Place Jules Janssen, Meudon, 92190, France

The Ulysses mission has produced corresponding sets of observations near two successive solar minima. A recent rapid scan from the solar equator to the polar caps in 2006 -07 provided solar wind and magnetic field measurements complementary to those in 1994-1995. Although the coronal wind is only 3 percent less fast, ion and electron densities decreased by 17 percent, causing solar wind dynamic pressure, pw, to be weaker by 20 percent while the open magnetic flux decreased by 36 percent. In the slow low latitude solar wind, pw has the same value and is independent of latitude along with the radial component of the magnetic pressure. The current solar cycle is also weaker including the polar magnetic field strength. These changes raise important questions that challenge our understanding of the solar -heliospheric connection. What causes related decreases in pw and the total open flux? How are they related to changes on the Sun? We will present an interpretation based on a recent model that relates these observables to the rate of emerging solar magnetic flux.

SH24A-03

Polar coronal holes properties during two solar minima

* von Steiger, R vsteiger@issibern.ch, International Space Science Institute, Hallerstrasse 6, Bern, 3012, Switzerland
Gloeckler, G gglo@umich.edu, Dept of AOSS, University of Michigan, 2455 Hayward St, Ann Arbor, MI 48109, United States
Zurbuchen, T H thomasz@umich.edu, Dept of AOSS, University of Michigan, 2455 Hayward St, Ann Arbor, MI 48109, United States

The current solar minimum appears to be peculiar in several respects. Although the first sunspots of the new solar cycle have begun to appear, solar activity still shows little incentive to rise even 12 years after the previous minimum. Moreover, the total solar irradiance is significantly lower than it was during the two previous minima. The Ulysses spacecraft flew over the polar regions of the Sun during both the previous and the current solar minimum. Using observations made with the SWICS sensor we investigate the properties of the fast streams emanating from the polar coronal holes, which dominate the heliosphere at times of minimum activity. During the solar minimum in 1994-95 a significant asymmetry between the north and the south polar hole was inferred from heavy ion charge states, with the north hole being the cooler one. During the polar passes in 2007-08 both coronal holes appear to be cooler than during the previous minimum, and once again there is a small N-S asymmetry. Interestingly, it seems to have the same sign as in 1994-95 although the magnetic polarity is now reversed. We will present and discuss several different element and charge state composition signatures to characterise the coronal hole properties in the current and the precious solar minimum.

SH24A-04

Multisatellite Observations of Interplanetary Field Enhancements

* Russell, C T ctrussel@igpp.ucla.edu, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 405 Hilgard Ave, LA, CA 90095, United States
Weimer, D R, National Institute of Aerospace,, Virginia Tech, Hampton, VA 23666, United States
Jian, L K, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 405 Hilgard Ave, LA, CA 90095, United States
Lai, H R, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 405 Hilgard Ave, LA, CA 90095, United States
Luhmann, J G, Space Sciences Laboratory, University of California, Berkeley, 7 Gauss Way, Berkeley, CA 94720, United States

Interplanetary Field Enhancements (IFEs) are magnetic structures in the solar wind that have a cusp-shaped maximum in the field strength with a strong current sheet often near the central peak. These structures generally last an hour or more. They have a tendency to be seen more often at specific ecliptic longitudes, have been identified on occasion with particular small solar system bodies (asteroid 2201 Oljato and comet 122P/ De Vico) and attributed to the interaction of the solar wind with charged dust. On occasion they are detected nearly simultaneously by several spacecraft. Multispacecraft detection have been made with PVO, Venera-13 and Venera-14; with ISEE 1 and ISEE 3 and more recently with STEREO A and B, ACE and Wind. In this paper we use a delay matching algorithm developed by D. Weimer on the IFE of December 24, 2006 detected by 4 spacecraft. While the IFE is crossing the four spacecraft separated in Y by 90 Re and in X by 160 Re the measured delay was close to the calculated advection time. Along the apparent center line of the event the delay was close to 4 minutes. This event together with previous events are consistent with IFEs being magnetic structures that are convecting outward from the Sun with nearly, but slightly slower than, the solar wind velocity. We need to understand the occurrence rate of such structures and their physical cause because if this hypothesis is true, they may be responsible for accelerating dust out of the inner solar system.

SH24A-05

Heliospheric Evolution of Small-Scale Magnetic Structures

* Cartwright, M L mcartwri@ucla.edu, University of California, Los Angeles, 595 Charles E. Young Drive East Box 951567, Los Angeles, CA 90095-156, United States
Moldwin, M B mmoldwin@igpp.ucla.edu, University of California, Los Angeles, 595 Charles E. Young Drive East Box 951567, Los Angeles, CA 90095-156, United States

Small-scale flux ropes have been observed in the solar wind at 1 AU with the WIND and ACE spacecraft and one event was observed at 5AU with Ulysses. These magnetic structures are similar to magnetic clouds and can be modeled with a constant-alpha force-free, cylindrically symmetric flux rope. They differ from magnetic clouds in that they have durations on the order of minutes up to a few hours, they lack an expansion signature, and they do not have a depressed proton temperature compared to the surrounding solar wind plasma. Magnetic clouds have been observed at several heliospheric distances and latitudes. There have been no systematic surveys for small-scale flux ropes at different heliospheric distances and latitudes. We will present a comprehensive small-scale flux rope survey between 1 and 5 AU using the Helios, PVO, WIND, ACE and Ulysses spacecraft to examine their occurrence frequency, properties and evolution.

SH24A-06

Bifurcated Current Sheets Produced by Magnetic Reconnection in the Solar Wind

* Gosling, J T jack.gosling@lasp.colorado.edu, Laboratory for Atmospheric and Space Physics University of Colorado, 1234 Innovation Drive, Boulder, CO 80303, United States
Szabo, A Adam.Szabo@nasa.gov, NASA Goddard Space Flight Center Heliospheric Physics Laboratory, Code 672 Bldg. 21, RM 234, Greenbelt, MD 20771, United States

We report observations from the Wind spacecraft of Petschek-like magnetic reconnection exhausts and thin current sheets in the solar wind on 19 and 20 November 2007, encompassing a solar wind disturbance driven by a magnetic cloud and followed by a corotating high-speed stream. We have identified an unusually large number (11) of reconnection exhausts in this 2-day interval using 3-s plasma and magnetic field data. Despite the relatively smooth, large-scale field rotation associated with the magnetic cloud, 5 of the exhausts occurred within the cloud; 3 of those exhausts were associated with extremely small (less than 18 deg) local field shear angles. All 11 exhausts contained double-step magnetic field rotations; such double-step rotations are called bifurcated current sheets since they result from the splitting of reconnecting current sheets as an after-effect of the reconnection process. We have also identified 27 current sheets in this 2-day interval that were too thin to be adequately resolved by the 3-s plasma measurement cadence. All of these thin current sheets were well resolved by the 92 ms magnetic field measurement. At least 3, and possibly 6, of these relatively thin current sheets had double-step magnetic field rotations, indicating the underlying current sheets had probably been disrupted by magnetic reconnection. Current sheets thinner than about 3 ion inertial lengths were not present in this data set. The relative lack of such ultra-thin current sheets in the solar wind in general suggests that such current sheets usually are quickly disrupted by magnetic reconnection.

SH24A-07

Backscattering of solar electron burst distributions

* Skoug, R M rskoug@lanl.gov, Los Alamos National Laboratory, Space Science and Applications, ISR-1, MS D466, Los Alamos, NM 87545, United States
Steinberg, J T jsteinberg@lanl.gov, Los Alamos National Laboratory, Space Science and Applications, ISR-1, MS D466, Los Alamos, NM 87545, United States
Anderson, B R brett@lanl.gov, Los Alamos National Laboratory, Space Science and Applications, ISR-1, MS D466, Los Alamos, NM 87545, United States
de Koning, C A Curt.A.deKoning@noaa.gov, NOAA, Space Environment Center, Boulder, CO 80305, United States
Gosling, J T jack.gosling@lasp.colorado.edu, University of Colorado, Laboratory for Atmospheric and Space Physics, Boulder, CO 80303, United States
McComas, D J dmccomas@swri.edu, Southwest Research Institute, Space Science and Engineering Division, San Antonio, TX 78228, United States

Solar electron bursts are frequently observed in the ACE/SWEPAM suprathermal electron measurements at energies below 1.4 keV. Approximately 1/3 of such events show backstreaming electrons, which are typically observed after a 1-2 hour delay following burst onset and travel back towards the Sun along the magnetic field direction. These backstreaming particles are likely produced by scattering of the solar electron burst particles beyond the ~1 AU spacecraft location. Our previous studies have shown that transient solar burst electrons exhibit a greater degree of pitch angle scattering than does the steady-state solar wind electron strahl, which could result in turning back of the burst electrons. From the delays observed between the onset of the bursts and the backstreaming, we infer that burst electrons are turned back sunward at a field-aligned distance of less than 1 AU beyond the spacecraft. In this study we compare the fluxes and energy spectra of backstreaming particles with those of the outgoing burst particles. In addition, we compare the angular widths of the two electron beams. These comparisons provide an indication of the relative importance of magnetic mirroring and focusing, scattering in pitch angle, and scattering in energy for determining the character of the observed electron distributions.

SH24A-08

Beam speeds and Langmuir wave growth in interplanetary type III bursts

* Bale, S D bale@ssl.berkeley.edu, University of California, Berkeley, Space Sciences Laboratory University of California, Berkeley, CA 94720,
Krucker, S krucker@ssl.berkeley.edu, University of California, Berkeley, Space Sciences Laboratory University of California, Berkeley, CA 94720,

We use measurements from the Wind spacecraft to study Langmuir wave growth in the source region of four separate in situ interplanetary type III bursts. Velocity distributions of energetic electron are used to compute the advection beam speed and growth rate (in the cold beam approximation) and these beam speeds and growth rates are compared with Langmuir wave activity and detailed features of the Langmuir wave spectra and waveforms. Knowledge of the electron beam speed lets us compute the resonant wavenumber and the expected spacecraft frequency of both daughter Langmuir waves and ion acoustic waves required for the parametric decay process. We find no strong evidence for decay products nor carrier splitting that is consistent with parametric decay. On the other hand, many of the Langmuir waveforms show 'polarization' signatures that may be consistent with linear mode conversion or trapped eigenmode interpretations of type III wave processes.

Authors (2008), Title, Eos Trans. AGU, 89(53), Fall Meet. Suppl., Abstract xxxxx-xx