A Study of Density Modulation Index in the Inner Heliospheric Solar Wind During Solar Cycle 23

Janardhan, P.; Ingale, Madhusudan; Subramanian, Prasad; Ananthakrishnan, S.; Tokumaru, M.; Fujiki, K.

doi:10.1088/0004-637x/795/1/69

Cited by 25 publications

(35 citation statements)

References 63 publications

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“…The significant reduction in the solar wind density in this cycle has already been reported from many earlier studies using in situ measurements (Jian et al, 2011;McComas et al, 2008McComas et al, , 2013Tokumaru et al, 2012). A long-term declining trend in the solar wind density turbulence level during Cycles 23-24 has also been found from the analysis of ISEE IPS observations (Bisoi et al, 2014;Janardhan et al, 2011Janardhan et al, , 2015Tokumaru et al, 2012), and this fact supports the rarefaction of the solar wind in this cycle. It should be noted here that the rarefaction of the solar wind is most remarkable for the speed range of the slow wind, though it takes place for all speed ranges (see Figure 13 of Tokumaru et al, 2012).…”

Section: Discussionsupporting

confidence: 86%

Rarefaction of the Very Slow (<350 km/s) Solar Wind in Cycle 24 Compared With Cycle 23

et al. 2018

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We investigate long‐term variation of the very slow solar wind (VSSW), whose speed is less than 350 km/s, during the period between 1997 and 2015, that is, Cycles 23 and 24, using interplanetary scintillation measurements, which enable determination of the global distribution of the flow speed and the electron density fluctuation level ΔNe. We find that the occurrence rate of VSSW increases in the maximum phase of the solar cycle, and it was mostly associated with high ΔNe in Cycle 23. However, the VSSW is found to be more associated with low ΔNe in Cycle 24. This fact is consistent with an increased occurrence of the low‐density VSSW observed in situ in Cycle 24. These facts, which are considered to be a manifestation of weak solar activity in this cycle, suggest that the VSSW has become significantly rarefied in recent years. We identify the source region of the VSSW on the photosphere using a potential field analysis and examine the magnetic field properties of the VSSW source. We find that the low‐ΔNe VSSW is associated with a smaller expansion factor and possibly a slightly weaker photospheric field strength than the high‐ΔNe VSSW. The results obtained here may suggest that more open magnetic field areas producing the VSSW in Cycle 24 are formed in the quiet Sun region, which is associated with a lower mass flux supply into the corona, than those in Cycle 23.

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Section: Discussionsupporting

confidence: 86%

Rarefaction of the Very Slow (<350 km/s) Solar Wind in Cycle 24 Compared With Cycle 23

et al. 2018

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“…This processes had started already at the onset of cycle 23 and is probably related to the reduced IMF turbulence, from spacial scales of a few hundred km (Bisoi et al, 2014) up to 10 7 km (Starodubtsev and Grigoryev, 2011), which affects the GCR propagation in the inner heliosphere.…”

Section: Discussionmentioning

confidence: 99%

“…Using solar wind in situ data (from the OMNI 2 database), Starodubtsev and Grigoryev (2011) found a sharp change in the residual solar wind turbulence level already after the onset of cycle 23, indicating a decrease of CR scattering in the inner heliosphere. Based on interplanetary scintillation observations between 1998and 2008, Bisoi et al (2014 reported a decrease of 8 % in the density modulation index in the inner heliosphere that might be related to the decrease in solar wind turbulence. Our results presented here support the evidence of a new dominant process that overcomes the drift in the last solar minimum.…”

Section: Solar Modulation Changesmentioning

confidence: 99%

“…For example, from comparing the HCS tilt angles with the NM counts of one ground-based station (Newark), Cliver, Richardson, and Ling (2013) first suggested (already in 2011) that the drift of charged GCR was not a dominant factor in the minimum of cycle 24, contrary to expectations. Recent studies found an association between the 2009 anomalous peak and higher values of the diffusion coefficient (e.g., Alania, Modzelewska, and Wawrzynczak, 2014;Nuntiyakul et al, 2014). Other studies also found evidence of the remission of the drift process in the solar modulation during the last minimum and suggested that changes in the solar wind turbulence might produce a diffusion-dominated solar modulation period (e.g., Starodubtsev and Grigoryev, 2011;Potgieter et al, 2014).…”

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confidence: 97%

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An Unusual Pattern of Cosmic-Ray Modulation During Solar Cycles 23 and 24

Pacini

Usoskin

2014

Sol Phys

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By means of an analysis of data from eight neutron monitor (NM) stations with different geomagnetic cutoff rigidities, we found an unusual latitudinal effect observed in the cosmic-ray (CR) modulation during the last solar cycles. Since the beginning of the ground-based cosmic-ray monitoring, it is known that the solar-cycle modulation is more evident in data from high latitude than from the medium and low latitudes, showing an expected geomagnetic cutoff rigidity effect. However, a more detailed look shows a new latitudinal effect in cycle 24: while the magnitude of the solar modulation in the low-latitude data remains the same for the last three solar minima, the last solar minimum caused a more intense peak in the polar NM data than in the previous cycles. After correcting the data for the geomagnetic changes of the period, we found an anomalous solar modulation in the last cycle. This suggests a weaker heliospheric modulation at low-energy particles (responsible for the NM counting in polar sites) now than in the previous cycles, while there is no significant difference of the modulation for the more energetic part of the CR spectrum. Our result can be associated with changes of the solar wind turbulence, which would corroborate some recent studies about the last solar minimum phase, and indicates that this new solar modulation feature is still present in the current solar maximum stage.

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“…• The density modulation index (ǫ Ne ≡ δN e /N e ) at the inner scale of the turbulent spectrum in the solar wind from 9 − 20 R ⊙ ranges from 1.9 ×10 −3 to 7.7 ×10 −3 (see Bisoi et al (2014) in the distance range 56-185 R ⊙ and 0.03 ǫ Ne 0.08 reported by Spangler & Spitler (2004) at 1 AU (215 R ⊙ ). The red circles in Figure 4 depict the modulation index as a function of heliocentric distance.…”

Section: Discussionmentioning

confidence: 96%

Turbulent Density Fluctuations and Proton Heating Rate in the Solar Wind from 9–20 R_⊙

Raja

Subramanian

Ramesh

et al. 2017

ApJ

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We obtain scatter-broadened images of the Crab nebula at 80 MHz as it transits through the inner solar wind in June 2016 and 2017. These images are anisotropic, with the major axis oriented perpendicular to the radially outward coronal magnetic field. Using these data, we deduce that the density modulation index (δN e /N e ) caused by turbulent density fluctuations in the solar wind ranges from 1.9 ×10 −3 to 7.7 ×10 −3 between 9 -20 R ⊙ . We also find that the heating rate of solar wind protons at these distances ranges from 2.2 × 10 −13 to 1.0 × 10 −11 erg cm −3 s −1 . On two occasions, the line of sight intercepted a coronal streamer. We find that the presence of the streamer approximately doubles the thickness of the scattering screen.

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A Study of Density Modulation Index in the Inner Heliospheric Solar Wind During Solar Cycle 23

Cited by 25 publications

References 63 publications

Rarefaction of the Very Slow (<350 km/s) Solar Wind in Cycle 24 Compared With Cycle 23

Rarefaction of the Very Slow (<350 km/s) Solar Wind in Cycle 24 Compared With Cycle 23

An Unusual Pattern of Cosmic-Ray Modulation During Solar Cycles 23 and 24

Turbulent Density Fluctuations and Proton Heating Rate in the Solar Wind from 9–20 R_⊙

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A Study of Density Modulation Index in the Inner Heliospheric Solar Wind During Solar Cycle 23

Cited by 25 publications

References 63 publications

Rarefaction of the Very Slow (<350 km/s) Solar Wind in Cycle 24 Compared With Cycle 23

Rarefaction of the Very Slow (<350 km/s) Solar Wind in Cycle 24 Compared With Cycle 23

An Unusual Pattern of Cosmic-Ray Modulation During Solar Cycles 23 and 24

Turbulent Density Fluctuations and Proton Heating Rate in the Solar Wind from 9–20 R⊙

Contact Info

Product

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Turbulent Density Fluctuations and Proton Heating Rate in the Solar Wind from 9–20 R_⊙