Madhulika Guhathakurta

Plasma Properties in Coronal Holes Derived from Measurements of Minor Ion Spectral Lines and Polarized White Light Intensity

Recent observations of the Lyα λ1216, Mg X λ625, and O VI λ1038 spectral lines carried out with the Ultraviolet Coronagraph Spectrometer (UVCS) on board SOHO at distances in the range 1.35-2.1 RS in the northern coronal hole are used to place limits on the turbulent wave motions of the background plasma and the thermal motions of the protons and Mg+9 and O+5 ions. Limits on the turbulent wave motion are estimated from the measured line widths and electron densities derived from white light coronagraph observations, assuming WKB approximation at radial distances covered by the observations. It is shown that the contribution of the turbulent wave motion to the widths of the measured spectral lines is small compared to thermal broadening. The observations show that the proton temperature slowly increases between 1.35 and 2.7 RS and does not exceed 3×10 K in that region. The temperature of the minor ions exceeds the proton temperature at all distances, but the temperatures are neither mass proportional nor mass-to -charge proportional. It is shown, for the first time, that collision times between protons and minor ions are small compared to the solar wind expansion times in the inner corona. At 1.35 RS the expansion time exceeds the proton Mg+9 collision time by more than an order of magnitude. Nevertheless, the temperature of the Mg ions is significantly larger than the proton temperature, which indicates that the heating mechanism has to act on timescales faster than minutes. When the expansion time starts to exceed the collision times a rapid increase of the O+5 ion spectral line width is seen. This indicates that the heavier and hotter ions lose energy to the protons as long as collision frequencies are high, and that the ion spectral line width increases rapidly as soon as this energy loss stops.

Flow properties of the solar wind derived from a two-fluid model with constraints from white light and in situ interplanetary observations

We derive the flow properties of the solar wind in coronal holes using a two-fluid model constrained by density profiles inferred from simultaneous space-based SPARTAN 201–01 and ground-based Mauna Loa White Light coronagraph observations, and by in situ interplanetary measurements. Also used as a guide is the hydrostatic temperature profile derived from the density gradient. Density profiles are inferred between 1.16 and 5.5 Rs, for two different density structures observed along the line of sight in a polar coronal hole. The model computations that fit remarkably well the empirical constraints yield a supersonic flow at 2.3 Rs for the less dense ambient coronal hole, and at 3.4 Rs for the denser structures. The novel result that emerges from these fits is a proton temperature twice as large as the electron temperature in the inner corona, reaching a peak of 2 × 106 K at 2 Rs.

Latitudinal variability of large-scale coronal temperature and its association with the density and the global magnetic field

In this paper we utilize the latitude distribution of the coronal temperature during the period 1984-1992 that was derived in a paper by Guhathakurta et al., 1993, utilizing ground-based intensity observations of the green (5303A Fe XIV) and red (6374A Fe X) coronal forbidden lines from the National Solar Observatory at Sacramento Peak, and estabish its association with the global magnetic field and the density distributions in the corona. A determination of plasma temperature, T, was estimated from the intensity ratio Fe X/Fe XIV (where T is inversely proportional to the ratio), since both emission lines come from ionized states of Fe, and the ratio is only weakly dependent on density. We observe that there is a large-scale organization of the inferred coronal temperature distribution that is associated with the large-scale, weak magnetic field structures and bright coronal features; this organization tends to persist through most of the magnetic activity cycle. These high-temperature structures exhibit time-space characteristics which are similar to those of the polar crown filaments. This distribution differs in spatial and temporal characterization from the traditional picture of sunspot and active region evolution over the range of the sunspot cycle, which are manifestations of the small-scale, strong magnetic field regions.

Flow properties of the solar wind obtained from white light data, Ulysses observations and a two-fluid model

We derive the flow properties of the solar wind using a two-fluid model constrained by the density gradients inferred from white light observations of a south polar coronal hole on 11 April 1993 during the SPARTAN 201-1 flight, and interplanetary observations, e.g. from Ulysses’ south polar passage. We present the results of model computations for which we get the best fit to these data. One of the main results of this study is that, for the same energy input to electrons and protons, the proton temperature can be significantly higher than the electron temperature in the inner corona. In addition, we show that different functional forms of the energy addition with the same total energy input can yield different solar wind parameters at 1AU.

Importance of white-light observations of the extended corona

White-light observations of the extended K-corona are an integral part of our understanding of the solar wind phenomenon and the coronal heating process. In this paper we outline why white-light observations of the corona are indispensible for establishing the connection between the sun and the inner/outer heliosphere and why inclusion of such an observational capability as a context instrument is crucial for the Solar Probe mission.

Observations and physical interpretations of coronal rays from white light, soft X-ray, red (FeX) and green (FeXIV) lines analyses

The purpose of this investigation is to determine the physical and morphological characteristics of the coronal rays as seen in white light and compare these structures to rays seen in soft X-ray, FeX (red) and FeXIV (green) line emissions. During the quiescent phase in solar activity, coronal rays were observed in white light both in the polar and equatorial regions by Spartan 201 and Mark-III. White light coronal rays have a brightness contrast of 10–40% compared to the adjacent structures. Polar coronal rays were observed in the red line emission and part of the time rays were also observed in soft X-ray and green line emission. The intensity contrast of the rays and the adjacent structures in these lines are ≈20–100%, in agreement with the white light observations. Polar rays observed in red line emission are designated as ‘cool’ rays while those observed in soft X-ray and green line emission are called ‘hot’ rays. Cool rays are a dominant feature of the polar corona during the quiescent phase of the ...

Density, Temperature and Magnetic Field Structure of High Latitude Corona

This paper explores the physical and morphological characteristics of the large-scale coronal structures such as polar coronal rays and high latitude streamers as seen in white-light and relates these structures to observations in soft X-ray (3–45 A), red (6374 A Fe X) and green (5303 A Fe XIV) line emissions to estimate their densities and temperatures. Analysis shows that polar rays can be characterized by at least two temperature classes. Cool (0.7–1.3 106 K) rays are a dominant feature of the polar corona during the quiescent phase of the solar cycle. The hot (1.8–2.6 106 K) rays when present form a small subset of the array of rays seen in white-light. Hot rays seem to emerge from the boundary of the polar coronal hole (polar crown filament belt). The location of the cool rays on the other hand can be on the boundary or inside the coronal hole. We do not always find a one to one correspondence between the polar rays observed in white-light versus those observed in XUV and visible emission lines. We find the emission line-ratio temperature to be high in the high latitude (> 45° N,S) coronal streamers with enhanced white-light emission. These streamers are located along a neutral line which separates the weak old cycle polar field from the weak new cycle high-latitude magnetic field of opposite polarity (Ap J (Letters), 471, 1, L69). This paper is an extended abstract for the Ap J Letters paper.