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Quiet-time F2-layer Disturbances: Morphology and some Formation Mechanisms Quiet-time F2-layer Disturbances: Morphology and some Formation Mechanisms Andrei Mikhailov Institute of Terrestrial Magnetism Ionosphere and Radio Wave Propagation (IZMIRAN) Russian Academy of Sciences

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Quiet-time Disturbances (Q-disturbances) is a special class of the F2-layer perturbations not related to geomagnetic activity 1.Their amplitude is comparable to moderate F2-layer storm effects resulted from increased geomagnetic activity 2. Their morphology is different from the morphology of usual storm-induced F2-layer disturbances 3. The formation mechanisms are also different (for Negative disturbances)

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Specification of Q-disturbances used in the analysis 1. (NmF2/NmF2 med – 1)x100% ≥ 40% If all 3-h ap indices were ≤ 7 for the preceding 24 hours 2. A 27-day NmF2 running median centered to the day in question 3. Only long-lasting, ≥ 3-h disturbances were considered 4. =NmF2/NmF2 med were used

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An example of negative Q-disturbance

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An example of positive Q-disturbance

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Seasonal occurrence frequency variation for negative usual and Q-disturbances

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Latitudinal occurrence frequency variation for Negative and Positive usual and Q-disturbances

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2-D distribution of NmF2/NmF2 med in the case of Positive Q-disturbance Notice latitudinal dependence for NmF2/NmF2 med variations

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2-D distribution of NmF2/NmF2 med in the case of Negative Q-disturbance Notice latitudinaly independent NmF2/NmF2 med variations

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Longitudinal variations of the NmF2/NmF2 med ratio under Q-disturbance events Steep front

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Physical Interpretation

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Positive and Negative Q-disturbances observed by Millstone Hill ISR

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lgNmF2 = 4/3 lg[O] - 2/3 lg + 1/2 lgTn Both Positive and Negative Q-disturbances are mainly due to [O] variations presumably resulted from the vertical gas motion in the whole thermosphere Retrieved Aeronomic Parameters at 300 km for Positive (Apr 11, 2000) and Negative (Apr 16, 2002) Q-disturb. Second line (Italic) – NRLMSISE-00 model values

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Negative daytime Q-disturbances correspond to low [O] and an enhanced poleward thermospheric wind. Opposite situation takes place for Positive Q-disturbances

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The 27-day running median level from which Positive and Negative Q-disturbances are counted from is the essential point in the mechanism of their formation. These median levels are different Average Ap over the 27-day period: 9.87±3.88 for Negative (35 cases) 16.96±6.78 for Positive (105 cases)

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The role of the foF2 median level in Negative and Positive Q-disturbances formation Quiet 27-day period results in relatively high median level Disturbed 27-day period results in relatively low median level M E D I A N Area of Positive Q-disturbances M E D I A N Area of Negative Q-disturbances

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Negative Q-disturbances occur under very low geomagnetic activity Monthly Ap indices and number (in brackets) of negative disturbances observed at Slough for three months and years around solar minimum.

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A sketch to illustrate the place of Q-disturbances on the Ap index scale

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The Ground State of the Thermosphere The Ground State corresponds to a very low geomagnetic activity with an unconstrained solar-driven (poleward) thermospheric circulation and low [O] at middle and subauroral latitudes. This follows from the model calculations by Rishbeth and Müller-Wodarg (1999)

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Negative and Positive Q-disturbances Formation Latitude Auroral zone Unconstrained solar-driven Vnx (Minimal geomagnetic activity) Neutral gas upwelling Latitudinal independent (Rishbeth and Müller- Wodarg, 1999) Latitude Auroral zone Solar-driven Vnx (Slightly enhanced geomagnetic activity) Neutral gas downwelling Latitudinal dependent (Rishbeth, 1998) [O] decrease [O] increase [O] decrease

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The differences between F2-layer Negative Storm-induced and Q-disturbances are in: 1. hmF2 variations 2. Ne(h) distribution

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Different hmF2 Variations 1. Magnetically storm-induced F2-layer changes always result in the hmF2 increase 2. Q-disturbance hmF2 always decreases A statistical check 169 (station/date) Negative Q-disturbance cases were analyzed: Average hmF2 deviation = 9.7 km Student parameter t = (The deviation is significant at any confidence level)

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Negative Q-disturbance on Apr 16 and Storm-induced on Apr 17-18, 2002 variations observed by Millstone Hill digisonde GG G-condition (NmF2 < NmF1) Incorrect hmF2 interpretation in such conditions

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Millstone Hill ISR observations for the period in question Millstone Hill ISR observations for the period in question ISR facility provides correct hmF2 under G-conditions

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Retrieved Aeronomic Parameters at 300 km for Apr 15, 16, 18, 2002 Apr 15, 16, 18, 2002 Italic – Millstone Hill Tex estimates Dates Tex log[O] 300 log 300 WW Apr 16/15, Apr 18/15, Variations of aeronomic parameters for the two pairs of dates

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A contribution of the main parameters to hmF2 variations All controlling parameters: log[O], log , Tn, and W are < 0 for Q-disturbance decreasing hmF2, [O] providing the main contribution and these parameter contributions are > 0 for Storm-induced disturbance increasing hmF2, the [O] contribution being small due to the competition between Tn increase and storm-induced thermospheric circulation

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Different Ne(h) distributions

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The difference in Ne(h) distributions results from different plasma temperatures, i.e. plasma scale heights Observed at Millstone Hill plasma temperatures, their gradient and plasma scale heights at 500 km for quiet and disturbed days. Date Te, K Ti, K d(Te + Ti)/dh, K/cm4.85e-54.49e-53.05e-56.00e-52.80e-5 Hp, km

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Conclusions

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Negative Q-disturbances 1. Negative Q-disturbances occur under the ground state of the thermosphere with low [O] and relatively strong poleward Vnx producing downward plasma drift W. 2. Both low [O] and downward W decrease NmF2 resulting in Negative Q-disturbances. 3. Clustering of the Negative Q-disturbances around winter solstice is related to the poleward Vnx, which is the strongest under such conditions.

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Negative Q-disturbances 4. In fact, the occurrence of Negative Q-disturbances is not related to any physical process, but depends on the NmF2 median level they are counted from. 5. For a magnetically disturbed month (low median level) the probability for Negative Q-disturbances to occur is lower. 6. For a magnetically quiet month (high median level) the probability for Negative Q-disturbances to occur is higher.

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Positive Q-disturbances 1. Positive Q-disturbances appear under slightly enhanced auroral activity when the high-latitude heating increases auroral activity when the high-latitude heating increases and damps the solar-driven poleward thermospheric and damps the solar-driven poleward thermospheric circulation. circulation. 2. This damping produces a downwelling of the neutral gas followed by the enrichment of the thermosphere gas followed by the enrichment of the thermosphere with atomic oxygen [O]. with atomic oxygen [O]. 3. The downwelling is expected to increase towards the auroral oval and this explains the increase with latitude auroral oval and this explains the increase with latitude of the amplitude and the occurrence frequency of of the amplitude and the occurrence frequency of Positive Q-disturbances. Positive Q-disturbances.

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Positive Q-disturbances 4. The damped poleward wind also reduces the downward plasma drift in the F2-region, thereby increasing NmF2. plasma drift in the F2-region, thereby increasing NmF2. 5. The Positive Q-disturbances just present the left-hand wing of the positive, long-duration F2-layer wing of the positive, long-duration F2-layer disturbances area on the Ap-index scale. disturbances area on the Ap-index scale. 6. The mechanism of both types of Positive disturbances is the same: the damped poleward circulation and neutral the same: the damped poleward circulation and neutral gas downwelling resulting in the [O] abundance gas downwelling resulting in the [O] abundance increase. increase.

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