Occultation of radio source PKS 2025-15 by comet Kohoutek (1973f)

Ananthakrishnan, S.; Bhandari, S. M.; Rao, A. Pramesh

doi:10.1007/bf00640353

Cited by 27 publications

(16 citation statements)

References 5 publications

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“…IPS observations thus enable one to probe solar wind electron density fluctuations of scale sizes from 10 to 100 km both in and out of the ecliptic [Pramesh Rao et al, 1974;Coles and Filice, 1985;Yamauchi et al, 1998;Fallows et al, 2008;Ananthakrishnan et al, 1995] and over a wide range of distances in the inner heliosphere [Janardhan et al, 1996]. In fact, IPS is sensitive to very small changes in the RMS electron density fluctuations (ΔN) and has even been exploited to study the fine scale structure in cometary ion tails during radio source occultations by cometary tail plasma [Ananthakrishnan et al, 1975;Janardhan et al, 1991Janardhan et al, , 1992 and to study extremely low density events at 1 AU referred to as solar wind disappearance events, when the average solar wind densities at 1 AU dropped to less than 0.1 particles cm −3 for extended periods of time [Balasubramanian et al, 2003;Janardhan et al, 2005Janardhan et al, , 2008aJanardhan et al, , 2008b.…”

Section: Solar Wind Microturbulencementioning

confidence: 99%

A 20 year decline in solar photospheric magnetic fields: Inner‐heliospheric signatures and possible implications

et al. 2015

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We report observations of a steady 20 year decline of solar photospheric fields at latitudes ≥45° starting from ∼1995. This prolonged and continuing decline, combined with the fact that cycle 24 is already past its peak, implies that magnetic fields are likely to continue to decline until ∼2020, the expected minimum of the ongoing solar cycle 24. In addition, interplanetary scintillation observations of the inner heliosphere for the period 1983–2013 and in the distance range 0.2–0.8 AU have also shown a similar and steady decline in solar wind microturbulence levels, in sync with the declining photospheric fields. Using the correlation between the polar field and heliospheric magnetic field (HMF) at solar minimum, we have estimated the value of the HMF in 2020 to be 3.9 (±0.6) nT and a floor value of the HMF of ∼3.2 (±0.4) nT. Given this floor value for the HMF, our analysis suggests that the estimated peak sunspot number for solar cycle 25 is likely to be 62 (±12).

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Section: Solar Wind Microturbulencementioning

confidence: 99%

A 20 year decline in solar photospheric magnetic fields: Inner‐heliospheric signatures and possible implications

et al. 2015

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“…Increases in scintillation were noted for Comet Kohoutek (Ananthakrishnan et al 1975) and Comet Halley (Alurkar et al 1986;Slee et al 1986). Conversely, Ananthakrishnan et al (1987) were unable to confirm an enhancement for Comet Halley while Hajvassiliou and Duffett-Smith (1987) reported negative results for a number of comet-tail occultations.…”

Section: Introductionmentioning

confidence: 96%

Radio Source Scintillations Through Comet Tails Revisited: Comet Wilson (1987)

Slee¹,

Bobra²,

Waldron³

et al. 1990

Aust. J. Phys.

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Observations of the quasars 0606-795 and 0637-752 as the tail of Comet Wilson swept across them on May 1 and May 2, 1987, showed a three-fold increase in scintillation index over that of nearby compact radio sources outside the tail. Two scintillation regimes have been identified: (1) small-scale turbulence of 10-40 km develops near the tail-axis; (2) large-scale turbulence of km is present in the off-axis transition region between the tail plasma and solar wind. At a distance 0·12 AU downstream from the nucleus the r.m.s. electron-density variation in these turbules is 4-8 cm-3 on axis and 0·8-1· 7 cm-3 in the transition region between the tail and the solar wind. The reported negative results from earlier comets are shown to be of doubtful significance.

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“…for € in excess of 60°, a positive result has been reported. During the first reported observations of enhancements in scintillation (Ananthakrishnan et al 1975), the solar elongation was only 20° but at 327 MHz this elongation is well into the region where weak interplanetary scattering prevails. Table 2 gives the parameters of the comet, while Table 3 gives the occulted and control source parameters along with the calculated scintillation index for different days and the corresponding scintillation index expected from the RKH model (Readhead et al 1978).…”

Section: Observations and Resultsmentioning

confidence: 97%

“…The question of whether cometary plasma contained in the tail of a comet can cause radio scintillations has been addressed several times, with positive results being reported in some cases (Ananthakrishnan et al 1975;Alurkar et al 1986;Slee et al 1986) and negative results in others (Ananthakrishnan et al 1987;Hajivassiliou and Duffett-Smith 1987). Recently, it has been shown conclusively (Slee et al 1990) that cometary plasma can contain a higher level of turbulence than the normal solar wind and give rise to enhanced scintillations.…”

Section: Introductionmentioning

confidence: 99%

Enhanced Radio Source Scintillation due to Comet Austin (1989c1)

Janardhan¹,

Alurkar²,

Bobra³

et al. 1991

Aust. J. Phys.

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Enhanced scintillations in the direction of the quasar 2204+29 (3C441) were observed on 13 May 1990 when the tail of Comet Austin passed in front of it. Comparison with previous observations at 103, 327 and 408 MHz of Comet Halley and at 408 MHz of Comet Wilson show that proper occultation geometry is essential for observing enhanced scintillations. It has been shown that the solar elongation € during such observations should be large, typically greater than 60· and in no case less than 30· at 103 MHz. At the time of the occultation the scintillation index (r.m.s./mean source flux) was greater than that expected for this source by a factor of 3. The r.m.s. electron density variation /IN, at a distance of 0·9 A.U. from the sun and 7·3· downstream of the nucleus, was found to be 6 cm-3 as compared with 1 cm-3 for the normal solar wind at 1 A.U. The corresponding scale sizes of the turbulence were found to be much finer than normally found in interplanetary scintillation (IPS) caused by the solar wind.

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Occultation of radio source PKS 2025-15 by comet Kohoutek (1973f)

Cited by 27 publications

References 5 publications

A 20 year decline in solar photospheric magnetic fields: Inner‐heliospheric signatures and possible implications

A 20 year decline in solar photospheric magnetic fields: Inner‐heliospheric signatures and possible implications

Radio Source Scintillations Through Comet Tails Revisited: Comet Wilson (1987)

Enhanced Radio Source Scintillation due to Comet Austin (1989c1)

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