Heliographic Coordinates Research Articles

Elimination of projection effects from vector magnetograms: The pre-flare configuration of active region AR 4474

We demonstrate a simple method of transforming vector magnetograms to heliographic coordinates. The merits of this transformation are illustrated using a vector magnetogram obtained with the MSFC vector magnetograph 80 minutes prior to a white light flare in active region AR 4474 on 25 April, 1984. The original magnetogram shows strong magnetic shear along the neutral line at both the flare site and a non-flaring site. The transformation of the magnetogram to heliographic coordinates shows that the elimination of projection effects results in a much shorter length of the sheared region at the non-flaring site than what is inferred from the image plane vector magnetogram. The length of the sheared region at the flare site is relatively less affected by the transformation.

Changes in measured vector magnetic fields when transformed into heliographic coordinates

In this paper we investigate the changes that occur in measured magnetic fields when they are transformed into a heliographic coordinate system. To carry out this investigation we took measurements of the vector magnetic field of an active region that was observed at 1/3 the solar radius from disk center and transformed the observed field into heliographic coordinates. We also examined differences in the calculated potential field that occur when the heliographic normal component of the field is used as the boundary condition rather than the observed line-of-sight component. The results of this analysis show (1) that the observed fields of sunspots more closely resemble the generally accepted picture of the distribution of umbral fields if they are displayed in heliographic coordinates, (2) that the differences in the potential calculations are less than 200 G in field strength and 20° in field azimuth outside sunspots, and (3) that differences in the two potential calculations in the sunspot areas are no more than 400 G in field strength but range from 60 to 80° in field azimuth in localized umbral areas.

Correcting Low-Frequency Solar Radio Source Positions for Ionospheric Refraction

We describe a method for reducing displacements in radio source positions caused by ionospheric refraction. Our method is an improvement on that used by Wild et al. (1959), who assumed all frequencies in the 40-70 MHz band came from the same position above the Sun and applied an f-2 correction for ionospheric refraction. Our method retains the dispersion of source position with frequency, which is inherent in the radio source, but allows for the f-2 ionospheric effect. We also discuss ways of estimating the absolute source positions from a knowledge of ionospheric density gradients. A typical example of ionospheric variations on solar records is shown in Figure 1(a). Here we have plotted the observed source positions on an f-2 scale. The measurements refer to a solar storm continuum burst which occurred on 1981 May 9 following an importance 2B flare at 08°N., 38°E., heliographic coordinates. From previous observations of such events we think it highly likely that the true source positions are stationary and displaced with decreasing frequency outwards along a line close to the radial direction above the flare. Figure 1(a) shows that: (a) the 160, 80 and 43 MHz source displacements vary as f-2; (b) there is a systematic slow drift of the source positions towards the south and east which increases with increasing hour angle; (c) superimposed on this steady drift is a quasi-periodic variation in source position with a period ~20 min.

Motions in the solar atmosphere associated with the white light flare of 11 July 1978

Series of white light heliograms and oft- and on-band Hα filtergrams have been obtained, with an average spatial resolution of 1″, to study the flare active McMath region 15403 on 11 July, 1978. A great number of accurate heliographic positions were determined for the umbrae, the white light flare patches and several bright Hα flare knots, as well as along the principal zero filament and an arch prominence. Using the measured heliographic coordinates of these objects their motions could be analyzed in some detail. The velocities of several different objects could be deduced from the coordinates. Since the heliocentric angle of the region was about 45°, the variation in apparent heliographic coordinates also enabled some variations in heights to be determined.

Progressive dipole waves as the constituents of the 22-year magnetic cycle

A pair of dipole waves propagating with constant phase velocity forward and backward relative to the solar rotation is suggested to explain the characteristic features of the field variation of the 22-year cycles. The interference of these waves results in a single dipole moving with varying phase velocity over the photosphere. The heliographic coordinates of the dipole axis are derived from the harmonic coefficients published for the Mount Wilson observational period 1959–1973. It is found that the dipole axis moves very slowly near the equator at the time of sunspot maximum whereas during the minimum it changes by 180° along a great circle in the direction of the rotation. During minimum the angular velocity of the axis is about ten times larger than during maximum and the poleward elongation of the axis is about 50°.

Identification of a solar X-ray source using D layer ionization behavior during an eclipse

Semi-quantitative reports of shortwave radio reception during the 12 November 1966 total solar eclipse have been used to determine the characteristics of a major source of D layer ionization. An effective electron depletion coefficient of 1.2 × 10−2 sec−1 was found empirically and used to reduce the data. Analysis of the radio absorption shows the source was located near heliographic coordinates, B = +7°, L = 340° and was probably less than 0′.5 of arc in diameter. At the time of the eclipse, the source accounted for 40% of the radio absorption on a single, vertical pass through the D layer. Preoccultation behavior of the signal strength is interpreted by assuming a portion of the source X-ray flux was reflected at grazing incidence from the limb of moon. For point sources, such reflections have specific chromatic characteristics which were used to derive a crude source spectrum in the 3–140 A range. X-ray absorption edges of N, O, Na, Be, and possible Mg, Ar, Ne, Ca, and K arising from the terrestrial atmosphere have been identified. A source temperature approximately twice that of the rest of the corona is indicated.

A geometrical aid in the determination of the disk positions of solar limb phenomena.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A geometrical aid in the determination of the disk positions of solar limb phenomena. Dodson, Helen W. ; Hedeman, E. Ruth Abstract An understanding of the significance of prominence activity may perhaps be acquired more readily if limb phenomena can be correlated more closely with associated features on the solar disk. The disk position corresponding to a limb feature is usually quite uncertain since prominences of great heights can be obseryed very far from the solar limb. The methods suggested in this paper have proved successful in determining the heliographic coordinates of certain prominence features recorded in the motion pictures taken at the McMath-Hulbert Observatory. Apparent height and heliographic latitude of stable solar prominences change with time because of solar rotation. If the poles of the axis of rotation are assumed to be on the limb, a prominence of height, h/R, of true heliographic latitude, B, and meridian distance, 6, will appear of height, H/i?, where H h R ~ ~~r1 - cos2Ocos2B - I, R being the solar radius. The difference between true and apparent height A/i? is given by AD (i +h)(1 - V1 - cos2Ocos2B). Furthermore tan B = sin 0 tan P where p is the observed heliographic latitude. For prominences within 300 of the limb, B differs from p by less than ~0, and in practical cases heliographic latitude can be considered as determined to this degree of accuracy by using the observed latitude p. The great uncertainty of disk position corresponding to limb observation lies in meridian distance and hence in heliographic longitude. For example, a prominence observed to be .o~i? in height, at latitude 400, may be located anywhere within a range of 600 of heliographic longitude depending on whether its true height is .o5i? or as much as .15i?. The rate of change of H/i? with 6, and hence with time, is different for different values of h/i?, 6, and B, and this variation can be used to determine the true height and heliographic longitude of stable features of prominences provided the observations cover an adequate time interval. In practice it is possible to use the set of curves of A/i? versus 6 for h/i? = .10 for all prominences for which h/i? ~ .15, provided 6 > 600 and an error in height of not more than 2000 km is permitted. In particular, this method has been applied successfully to prominences photographed on August 22-24, 1946, September 18-21, 1946, and July 1-3, 1947, with the tower telescope of the McMath-Hulbert Observatory. In each case it has been possible to identify the several features of a semi-active prominence with corresponding portions of a filament and to study the relationship of active and stable portions of the prominence to spots and faculous regions on the disk. McMath-Hulbert Observatory, University of Michigan, Lake Angelus, Pontiac, Mich. Publication: The Astronomical Journal Pub Date: February 1948 DOI: 10.1086/106067 Bibcode: 1948AJ.....53..111D full text sources ADS |

A Simple Geometrical Construction for determining the Heliographic Co-ordinates of Sun-spots

A Simple Geometrical Construction for determining the Heliographic Co-ordinates of Sun-spots

The Heliographic Coordinates of Sun-spots and Facul on the Stonyhurst Drawings

The Heliographic Coordinates of Sun-spots and Facul on the Stonyhurst Drawings