29 research outputs found
Allen Telescope Array Multi-Frequency Observations of the Sun
We present the first observations of the Sun with the Allen Telescope Array
(ATA). We used up to six frequencies, from 1.43 to 6 GHz, and baselines from 6
to 300 m. To our knowledge, these are the first simultaneous multifrequency
full-Sun maps obtained at microwave frequencies without mosaicing. The
observations took place when the Sun was relatively quiet, although at least
one active region was present each time. We present multi-frequency flux
budgets for each sources on the Sun. Outside of active regions, assuming
optically thin bremsstrahlung (free--free) coronal emission on top of an
optically thick ~10 000 K chromosphere, the multi-frequency information can be
condensed into a single, frequency-independent, "coronal bremsstrahlung
contribution function" [EM/sqrt(T)] map. This technique allows the separation
of the physics of emission as well as a measurement of the density structure of
the corona. Deviations from this simple relationship usually indicate the
presence of an additional gyroresonance-emission component, as is typical in
active regions.Comment: 16 pages, 11 figures. Accepted for publication in Solar Physic
Heliolatitude and time variations of solar wind structure from in situ measurements and interplanetary scintillation observations
The 3D structure of solar wind and its evolution in time is needed for
heliospheric modeling and interpretation of energetic neutral atoms
observations. We present a model to retrieve the solar wind structure in
heliolatitude and time using all available and complementary data sources. We
determine the heliolatitude structure of solar wind speed on a yearly time grid
over the past 1.5 solar cycles based on remote-sensing observations of
interplanetary scintillations, in situ out-of-ecliptic measurements from
Ulysses, and in situ in-ecliptic measurements from the OMNI-2 database. Since
the in situ information on the solar wind density structure out of ecliptic is
not available apart from the Ulysses data, we derive correlation formulae
between solar wind speed and density and use the information on the solar wind
speed from interplanetary scintillation observations to retrieve the 3D
structure of solar wind density. With the variations of solar wind density and
speed in time and heliolatitude available we calculate variations in solar wind
flux, dynamic pressure and charge exchange rate in the approximation of
stationary H atoms.Comment: Accepted for publication in Solar Physic
Solar parameters for modeling interplanetary background
The goal of the Fully Online Datacenter of Ultraviolet Emissions (FONDUE)
Working Team of the International Space Science Institute in Bern, Switzerland,
was to establish a common calibration of various UV and EUV heliospheric
observations, both spectroscopic and photometric. Realization of this goal
required an up-to-date model of spatial distribution of neutral interstellar
hydrogen in the heliosphere, and to that end, a credible model of the radiation
pressure and ionization processes was needed. This chapter describes the solar
factors shaping the distribution of neutral interstellar H in the heliosphere.
Presented are the solar Lyman-alpha flux and the solar Lyman-alpha resonant
radiation pressure force acting on neutral H atoms in the heliosphere, solar
EUV radiation and the photoionization of heliospheric hydrogen, and their
evolution in time and the still hypothetical variation with heliolatitude.
Further, solar wind and its evolution with solar activity is presented in the
context of the charge exchange ionization of heliospheric hydrogen, and in the
context of dynamic pressure variations. Also the electron ionization and its
variation with time, heliolatitude, and solar distance is presented. After a
review of all of those topics, we present an interim model of solar wind and
the other solar factors based on up-to-date in situ and remote sensing
observations of solar wind. Results of this effort will further be utilised to
improve on the model of solar wind evolution, which will be an invaluable asset
in all heliospheric measurements, including, among others, the observations of
Energetic Neutral Atoms by the Interstellar Boundary Explorer (IBEX).Comment: Chapter 2 in the planned "Cross-Calibration of Past and Present Far
UV Spectra of Solar System Objects and the Heliosphere", ISSI Scientific
Report No 12, ed. R.M. Bonnet, E. Quemerais, M. Snow, Springe